Make sure you know the rules. You essentially referee the game. You don't need to memorize them but you should know them well enough to make the game go smoothly… and don't harp on the rules too much. Put enjoyment of the game first and rules second! You also must keep in mind that you are probably going to mess up, but that is okay. You'll do better next time and each time you play, you will learn from your experiences. Remember, you aren't trying to "win," you are trying to create an interesting and compelling story with the rest of your party.
"If the characters hear a noise behind them, try to avoid saying “you all hear a heavy growling behind you and turn around to see what it is." Even though the characters are almost definitely going to turn around, you don’t have the right to..."
"The remarkability of crits in a game is often represented by additional damage or bumbling failures. The problem with making critical successes amazing experiences and critical failures a terrible one for players means that as a Dungeon Master you’re..."