First time game master guide.

What is it like?

So, before we delve into the nitty gritty let's first talk about what being a GM is like. It'll help with deciding whether you even want to be a GM after all.
Being a GM is a wonderful experience. You get to create your own story universe and have people play within it. Their enjoyment of your world is a true delight and this can be incredibly exhilarating, making you want GM more and more. But being a GM can also nerve wrecking as your creations are pretty much under direct scrutiny.

If this sounds absolutely terrifying do not worry, it's really not that bad. While your players do look to you for a fun story and just an overall great experience, players don't generally expect you to give them the most amazing story ever told by anyone ever. If you're able to engage them and provide them with a few (or many) hours of entertainment they will enjoy you as a GM. Plus most players are very lenient towards new GMs. Which brings me to my next point on the importance of playing with the right players.

Choose your players carefully

By far the biggest factor in a successful and enjoyable playing experience is finding the right group to play with. This goes both ways of course, you need to find players that match your style and players need to find a GM that fits theirs.
I know this is incredibly difficult in some cases, some simply don't live in an area with many tabletop role-players, so sometimes you do have to compromise, but that's perfectly fine and just part of life. If all else fails there's always plenty of ways to play tabletop role-playing games online.

To find the right players you need to ask yourself a few questions like 'What do I want from my campaign?', 'What kind of players would best fit the way I GM?' or 'Am I capable of dealing with a bad player?'. There's plenty more questions you could ask yourself, not all of them will matter, but it's a good exploration process.
Questions like 'Am I capable of dealing with a bad player?' are important though. If you aren't able to deal with a player who ends up ruining the game for others you'll have to make sure you avoid such players even more than usual, but that's easier said than done. Alternatively you could have somebody in the group who is capable of doing so and use them as a support.
People are generally all looking for a good time though, so bad players are usually pretty rare and those that do exist often only need a small nudge in the right direction to show their play style isn't all that great for the enjoyment of others.

Besides asking yourself questions you also need to ask your players questions. Questions like 'What do you expect from a campaign?', 'What kind of character do you enjoy playing?', 'How much decision making do you want to have?' and so on. Depending on what your players want you can either alter your story, alter the way you GM, look for different players or look for a different gaming system.

Choose your gaming system

Picking the right gaming system is pretty important, especially for longer running campaigns. But it's not as important as picking the right players since you can usually change a system between sessions relatively easily, but changing players is not easy at all. Unless you're comfortable with just dropping people and replacing them with others, which you hopefully aren't.

There are countless systems you can play. There are rules-heavy systems, very light systems, systems that rely entirely on dice rolls, systems that require no dice rolls at all, systems that depend entirely on the GM, systems that depend partially on a GM and partially on player input and so on.
I generally find game systems to fall in either one of two categories. The first are those that rely more on game mechanics and GMs to determine how the story flows. Dungeons & Dragons, Pathfinder, Shadowrun and other similar games are examples of this. In these games you state your intentions, roll the dice and the dice will determine whether you fail or not. The GM will then determine how your success or failure influences the story.

The other type relies more on player input and either does away with dice entirely or diminishes how much they can alter the game. Fiasco and Fate are examples of this. In games like these it's often the players who decide a lot of what happens. In fact the players often have a lot of control over the story universe and can directly influence parts of it, often by merely stating something, which then becomes fact according to the rules of these systems.

Many systems have a mix of these two elements of course, but depending on how much input you want to give your players it's wise to pick a system accordingly. More player input could mean less work for you and thus less pressure, but it could also throw your entire story universe for a loop when a player changes something completely.

Choose your campaign and setting

So you've found your players, you've found your game, now all you need is to find or create a campaign. This is where the fun starts for most GMs, it's the part where you get to be creative, plan encounters, create traps or puzzles and just let your imagination go wild.
On the other hand this is where things get overwhelming for some GMs, both new and old. You may be faced with an overwhelming amount of decisions to make, like 'How many monsters do I put in?', 'How hard should my encounters be?', 'What do I do about loot?', 'How will I keep track of everything?' and so on.

Again, don't worry. I go through many of these aspects both in this guide and in separate guides, but let's not get ahead of ourselves just yet, we're still focused on getting a campaign.
If you're uncertain about your abilities to create a fun campaign or just don't have the time there are plenty of places online which provide pre-generated (pregen) campaigns and even pregen characters (in case your players prefer pregens as well). Not only are these pregens a helpful way to get the hang of being a GM without all the hassles of creating everything yourself, they're usually also proven to be pretty entertaining.

On the other hand it's far more rewarding when a story you created goes well and the vast majority do. So if you're able to create a campaign, either from scratch or loosely based on an existing story, I strongly urge you to do so. Besides the satisfaction it brings you it's also a great learning experience.

How much you need to create

This is one major issue most new GMs face when they create their own campaign. How much story fits inside a session? How many fights should I put inside a session? How strong should my monsters be? What about X and Y and Z?
The answer to all of those is: It depends. "Gee, thanks Emily, that's very helpful.". Bare with me here. The amount of fights you need depends on how much your players like fights as opposed to social encounters, puzzles or exploration for example. How much story fits inside a session depends on how fast your players play through a session and how descriptive you are in describing scenes. How strong your monsters should be depends on the party size, the level of the characters, the experience of the players and their desire for an intense fight or just a regular hack and slash game.
Then there's also the element of things never going according to plan. Your players will deviate from the path you've set them on and your story will change with every session.

"Great. So I have no clue what to prepare or how much to prepare.". Well, that's only partially true. What do you know? You know your campaign, so you know at least some of the elements you absolutely need. You also know roughly how long the campaign's supposed to last. That's all you really need. Take all the elements you absolutely need, now create some elements that could fit anywhere and add some elements that could fit in specific places, but aren't mandatory. Now depending on how the story flows you can add or leave out these elements on the fly.

Here's an example: Say the party's goal is to infiltrate a castle, rescue a captive and make it out alive. Well, what do we absolutely need?
- Mission briefing.
- Infiltrate the castle.
- Find the captive.
- Get away with the captive.

That's pretty much all you absolutely need. But of course just having these 4 points alone will generally make for a very short session, so what can we add?
The obvious thing to add is a security force in front of the castle and one guarding the captive as well. What else? Well, the mission briefing could be a social encounter instead of a letter. Perhaps this social encounter is in a town a little while away, so they'll have to travel through a forest, which could potentially have bandits along the way. What else? Maybe the captive is held by a (mini) boss or maybe a security system will trigger, locking doors through magic and forcing the group to either disable the system per door or find a new route.
So eventually you end up with something along the lines of this:
- Encounter NPC in town and gain information.
- Travel through the forest to the castle.
- Encounter bandits/wild animals in the forest.
- Make it to the castle. Do reconnaissance.
- Infiltrate the castle by either killing the guards or avoiding them somehow.
- Find the location of the captive, perhaps through interrogating a guard or pickpocketing a map or document.
- Find the captive, kill or otherwise get rid of anybody potentially guarding the captive.
- Find your way out of the castle with the rescued captive while dealing with whatever's in the way.
- Make your way through the forest again, potentially dealing with threats.
- Collect your reward plus gain a potential lead for the next session/adventure.

With a simple list like this you can easily throw in an encounter when the party is going too fast or leave something out when they're going too slow. For example, say the party is particularly slow getting to the NPC at the start and gaining the information they need, in this case you don't add an encounter while they travel through the forest.
Now let's assume they get rid of the guards incredibly quickly with some lucky dice rolls or just clever fighting and they make it to the captive in no time, throw in the boss character. What if they're killing the boss pretty quickly as well? Make the boss yell for help before the party can kill them and they thus summon the entire guard force. Now the party is forced to flee while fighting of one guard after the other until they make it out of the castle. Once they're out of the castle and into the relative safety of the forest they can talk to their rescued captive and gain more background information.
Yikes, did it all still take a little shorter than expected? Make the party sleep in the forest. It's too dark and too rainy to safely make your way through the forest right now. Make them decide who's taking first watch. Perhaps throw in a lone wolf or a pack of wolves if you're really short on time.

Every story can be stretched out or thinned depending on what it needs. So if by the time half of your planned session time has gone by and you're only 30% through your checklist, maybe skip a few of the optional parts to speed it along or slow it down even further and stretch the story across two sessions, giving you time in between sessions to further plan the second part.
Do try to make sure each optional encounter is different though. It's no fun fighting a group of creatures three times in a row, even if they are different creatures. The first could be a regular fight, the second could be a social encounter and the third could be an overwhelming force the party has no choice but to flee for example. There's plenty of variety to play with and it helps keep everything fresh and engaging.

You're not a dictator

Now that we've covered 4 major elements of a successful game let's get one other major element out of the way: You're not a dictator here, your wishes and desires are not law. This may sound a little extreme, but it helps to get the point across.
Many GMs make the mistake of thinking they're the ones in charge of everything and what they want to happen happens. While this can definitely be true it usually makes for an awful experience for the players.

Remember, you're all playing to have fun and to a large degree tabletop role-playing is a co-op game. Yes, you create the world, but the players play in it. If they say their character does something and it is within the rules and possibilities of that universe let them do it.
Do they want to go left when you want them to go right? You let them. Do they want to kill an NPC that's supposed to be crucial to the story later? You let them. Do they want to pass on an opportunity to help somebody out and instead go to the next town or just sit in a tavern and drink? Guess what? That's right, you let them.

This may sound strange, especially to a new GM, but trust me on this one when I say that this not only leads to a far more enjoyable experience for both the players and yourself, it also helps with making the world feel more real, helps the players feel more immersed and it often leads to some of the best gaming moments you'll remember later. Besides, their decisions could end up biting them in their butts later, if you so choose. Just don't be vindictive.

Your players know nothing

Remember this and remember it well. Your players might know their characters inside out, they might know all the rules of your chosen game system and they might even know you as a person very well and thus expect you to do and create certain elements, but they don't know what you have planned for them until you reveal it.
Why does this matter? It matters because it allows you to change anything and everything on the fly. Well, anything and everything that hasn't been established already.

Did your players not go into that final ominous looking room in the cave that would've revealed an important secret altar? Well, guess what? That altar is now in the forest they're travelling through on their way back.
Did your players kill an NPC that would later turn out to be the leader of an underground resistance force dedicated to overthrow the king? Well, guess what? It turns out that character had a son and that son is now in charge and he is infuriated, plus your players have pretty much declared war on him by killing his father.

My point is that just because your players throw some of your plans for a loop it doesn't mean you can't just go ahead with them anyway. The trick is to make them think all their decisions matter, which they do to some extent, but ultimately you are often still capable of going ahead with the plans, albeit an altered form of those plans. So go with the flow and change things when needed. Games rarely, if ever, go according to plan, so don't plan too much.

Know the rules

So far we've covered a lot, but there are a few more things to know before venturing out on your first GM session and one of those is to know the rules. It'll help greatly with making everything go smooth. "But Emily, the rule book is hundreds of pages long, do I really need to learn everything?". No. It'll help immensely, but ultimately you only have to know that which applies to your campaign. What does that mean though?

It means know the basics of how the game works (how actions work, which dice to roll when, what abilities to use, etc.), know what characters your players have and everything related to them and know how to deal with anything you've planned for the first session. If you've got traps know how a trap can be resolved. If you have an opportunity for characters to shop know how shopping works and what things are generally worth.
Also try and memorize things that might happen based on what characters could potentially do. There's no way to know for sure and there's no way to plan for everything, but every bit helps.

In the end it won't matter a huge deal if you don't know all the rules inside out though, so don't worry too much. Maybe one of your players does know all the rules and can help you or, if the players know just as much or less than you, ignore the rules and go with what you feel is right.
Remember that while players do expect you to know the rules to some extent, if you're up front about your knowledge they'll know what to expect. Plus nobody can realistically expect a GM to know every single rule and how to apply them in every single situation, at least not in the largest systems in which the rules can often be interpreted multiple ways.

Rules vs. Fun

A game in which the GM has to pause every 5 minutes to look something up isn't very fun. The flow of the story and game is broken and everything's just slowly becoming more and more frustrating for everybody. So don't look up the rules when you don't really have to. In many cases you can make a decision yourself and look things up after the game and remember it for future sessions. Make sure your players know this to avoid confusion in the future though.

The exception to this rule is when it comes to anything that might alter something significantly. In most cases these are 'Can I change/do X for/instead of Y?' related issues. All other issues are often either fairly minor or could be made to be temporary until further clarification between two sessions.
In some cases the rules aren't very clear either. It happens more often than you might think. In those cases you as the GM decide which way the rules go and it's best to let it go whichever way's more fun.

Final tips

You should be able to run your first game now and be pretty successful. But I'll leave you with a few more tips for good measure:
- Don't overdo anything. Don't plan too much, don't create too many encounters and don't worry too much.
- Be organized. It helps avoid silent moments and thus keeps the flow going.
- The answer is usually yes. 'Is there a cart nearby?'. Yes there is (now). 'Does the bar sell strong alcohols?' Yes, it does (now). Players often have a plan when they ask these questions, so see where they take you. But at the same time don't say yes to all the questions that make things easier. 'Is there a keyhole in this door?'. Nope, you'll have to find another way to find out what's behind it. Go ahead, open it. It's clearly not locked and surely there's nothing dangerous behind it, right?
- Take notes. Did a new NPC pop up? Write down their name so you know who they are later. Did a new idea pop up mid session? Write it down. Did you realize a mistake or are you uncertain about a specific rule? Guess what? That's right, write it down.
- You'll make mistakes. "No, I wo-", you will make mistakes. Accept it now and it'll make things much easier. Making mistakes is just part of it and it's no big deal at all. Players often don't even notice anyway.
- Learn from your mistakes. There's no better way to improve.
- It's only a game. No matter what system you play, no matter how big your campaign and no matter how many players you have, in the end it's just a game and people are here to have fun. So relax and enjoy yourself.