Being the GM can be a daunting task, especially when you lack experience. Here is some advice to get you started.
GMing is supposed to be fun! You will be at your most creative when you are relaxed and well-prepared. Try to to enjoy yourself and don't worry about a "perfect" performance. You are playing for a forgiving audience.
The most important thing that happens at the table are the consequences of what the PCs do. Do not try extraordinary measures to keep the PCs on track. Allow players time to debate strategy, organize their resources, and plan; a good RPG challenges the players to be creative.
There is no plotEdit
It is a good idea to have fairly detailed idea of what you would like to happen. It is an even better idea to toss this plot aside without remorse or hesitation when the players come up with something on their own. No plot survices contact with the players.
Remember you are a player, tooEdit
Respect other players as equal partners in creating the game. While the GM has extra tasks in preparing for the game, and in many groups is also mostly responsible for designing the game and guiding the players, the GM should also have the opportunity to have fun. Don't be afraid to laugh, play a role, or relax control in order to fix problems. Listen to your other players; they are your most important feedback, and the only audience you have besides yourself.
Understand the differences between games and fictionEdit
Some genre conventions are hard to pull off without the cooperation of the players; recruit them, or modify the trope so that it requires less consent and causes less anxiety. Realize that PCs have minds of their own. Recognize the players as equal, if not more important, authors in their own right.
The more you do ahead of time and the more you rehearse, the more relaxed, spontaneous, and creative your will be. At the same time, don't waste time detailing complicated things that may or may not happen. The best preparation is to plan for the unexpected. While that may sound paradoxical, you can certainly prepare a flexible approach for dealing with the situation where players go off the map.
If things do not go as planned, go with the flow. Even if the PCs head into disaster, treat the experience as an interesting story in its own right. Move things around. Relocate events and organizations. Juggle planned encounters. Take advantage of premade challenges in the rules such as monsters and NPCs. Wing it. Don't be afraid to ask for a quick break to brainstorm, or to cover for a lack of preparation by making noises to end the session fifteen minutes early.
Don't be afraid to save the PCs's bacon if it furthers the game, or to kill a PC if it creates real interest. Let minor NPCs suddenly become significant allies or enemies to the PCs. Did the PCs fail to save the world? So be it! The world is now a shattered wreck and they must do what they can to survive.
Keep it simple at firstEdit
Seriously, a few basic events, some NPCs, and PCs is all you need to have a fun Friday night. Spend time preparing a few things well instead of trying to master the whole thing at once.