Hi there! Looking to run a game? A little nervous? It's ok to be nervous. It means that you care. You care about doing a good job, that everyone will have a good time. When you step into the Games/Dungeon Master's shoes, you are putting yourself "out there" in a way, and that can be very scary.
1 - The first thing that you really need to understand is that everyone there just wants to play a game. They want to escape into a world of fantasy (or whatever genre you're playing). And undoubtly, the players will know it's your first time running a game, so they'll be very forgiving.
2 - Don't worry about knowing every single rule in the book before you start. I've been playing and running D&D for over 7 years now and I couldn't tell you the exact rules for chase scenes and I never remember how the jumping mechanics work. And if you have a veteran at your table, they're a good resource to have. But remember, YOU are the GM and make all the final calls when anything is in question.
3 - When establishing a game, make sure you understand what everyone is ok with on the onset. I touched on this in my 10 guidelines for role-playing table ettiquette. Session Zero is a great place to start. You can find out what kind of game everyone wants to play and what they're comfortable with.
4 - This may get me into a little trouble, but I'd honestly suggest you try not to say "No." There's an improv game called "Yes, and..." where actors build on eachother's ideas. Roleplaying games are very similar, in that you are building a story together. If you find yourself saying "No" to your players a lot, I would examine whether you are allowing player agency, or if you are railroading your players into Your Story. There will be times where what they want to do it a little crazy, but that's where the standard DM line of "You can try..." comes in.
5 - Have an understanding of character creation for whatever system you're using. I say this so that you can help support your players with what they want to do. But also go over their character sheets so you don't have any accidental gods running around ruining the other PCs fun. Work with the players and if you have questions or concerns, you can always check in with other more experienced GM/DMs to get their opinions.
There are tonnes more we could put here, but instead I'll point you in a couple directions. We run a GMs Chat on Twitch every-other Sunday at 4pm GMT. We talk over a range of topics and happily take questions from the chat, our discord community and emails. You can catch replays on our youtube channel. On our discord server, we have a channels for "Running the Game," "Virtual-Tabletop Tips and Tricks," as well as general, "Questions and Answers." A link to join can be found on the front of our home page or events page.
If you're looking to improve your GM skills, or maybe wanting to start but don't feel confident, then we'd love to help you! Next week, we're starting our GM Academy (requiring an investment), which is about 8 weeks long and includes 4 sessions of practical lessons (drop-ins available for these), building and running encounters in a safe and supportive environment and ending with running one-shots for eachother. Each enrolled student will have their own experienced Mentor as well. We're very excited to roll this out and look forward to helping another group of GMs run their own games and tell amazing stories. If you'd like more information, please check out the GM Academy page on our website.