Becoming a better game master

Read

The most obvious way to improve your GM skills is to simply read up on it, which, since you're reading this guide, you're doing right now. Hooray!
There are countless books and articles specifically aimed at helping people become a better GM, but finding the right one for you can definitely be a struggle. But books and articles don't have to be the only reading you do, there are plenty of forums and other sites with experiences, transcripts, questions, and struggles of other people, both players and GMs alike. They can often be a valuable resource of information, as personal experiences are generally just great moments anybody can learn from.

There's more than just articles, forums, and books aimed at GMs you can read. There's plenty of other relevant fields you could explore. Guides on writing stories, articles on creating compelling characters, books on improvisation, and so on.
There's always a few lessons you can learn from all sorts of fields in life. Adapting those lessons and experiences can be a fun challenge in and of itself, but they can also make for great campaign setups.

Ask

There's never any harm in asking for help. You could do it on dedicated forums, by sending a fellow GM a message, or by simply asking your players for feedback. The last option is definitely the more intimidating one, but players are usually happy to help you improve, it means their game will improve as well after all.
But bouncing around ideas with a fellow GM can be very insightful as well. They may have figured out ways to overcome one problem, you might have found others, and with your combined experiences and knowledge you can often find new elements to throw at your players.

There's one more person you can ask questions from though, and that person is yourself. Self-analysis is a very helpful skill to have. During and after each game you can usually identify the moments game play was a little slow, moments you may have had to fumble pieces, or perhaps moments in which your organisation failed a little. Either way there's a lot you can learn from yourself and your mistakes, even if the mistakes went completely unnoticed by your players. Not every mistake is worth worrying over of course, but if you do want to improve your GM skills, self-analysis is a good place to start.

Watch

There are dozens of live streams, video series, and other visual forms of both entertainment and education you can use to improve your GM skills. Watching better or just different GMs run their games, looking up a video tutorial on a specific topic, and even things like improv shows can be a helpful tool. The advantages of these are that you get to see it in action, and, if you're watching recordings, you can re-watch them easily to try and figure out their methods.
Of course the downside is that their methods might not always be as clear, and those who are really good at anything usually make it seem effortless. Still, take it one step at a time, focus on a specific element of their game running method, and you'll be able to pick up bits and pieces easily.

Listen

Instead of watching video series you could also either simply listen to them, or listen to one of the many podcasts out there. While they aren't always as good a learning device as videos, as they obviously lack the visual elements, they can still be very helpful for some elements part of running a game, like pacing, non-player character interactions, and dealing with player questions or problems.

Part of listening is also learning to listen to your players during the game, and picking up on ques and hints the players might be throwing at you. Whenever a game's going slow, devolves into buffoonery, or is otherwise obstructed in some form, some of your players might be throwing hints at you to move things forward again.
In other times part of the game might be dragging on for too long, and hints of this can be found in the body language of the players, as well as their way of speaking. So listen and look for moments in which your players aren't fully focused on the game, then figure out why they aren't.

Do

Lastly, there's always the method of simply practicing, and it's a very effective method. Simply playing games will cause you to run into your weaknesses as a GM at one point or another, which will often require them to be overcome or avoided in some way right in that moment. You might not be able to overcome these obstacles in the most efficient way immediately, but they're great learning opportunities. Players won't always notice you're struggling either, so don't worry about that. If they do notice, ask for help or input, or simply let them know you're working on becoming a better GM, they'll usually understand. Besides, if you took it upon yourself to a run a game for them, they'll very likely be grateful for it, and very willing to cooperate.

The only way to put into use what you've learned is to actually try it out. Not every lesson you learn will work for you, each GM is different, and each group is different. This is another reason why it's important to just play, as it'll allow you to get to know your group better, as well as your own GM style. This in turn helps you with improving and tailoring your GM skills to your group's specific needs. Eventually you'll be able to adapt to different groups with ease. But remember, everybody's just out to have fun with friends, make sure the fun isn't drained for you or others.