1) See it before you call it.
2) Really ... see it before you call it. Get the number and be sure. No peripheral vision fouls.
3) Don't get caught up in the play - you can't see anything if you are in the middle of the action.
4) Don't get stuck focusing on the ball - 90% of the action is NEAR the ball, but not AT the ball - watch the area you're responsible for. Don't look up to watch a ball in flight - focus on the players.
5) Look good. You can stink your first several games, and if you look good doing it (meaning being dressed right, and not sloppily, hustling, and looking like you should be there), you'll be all right. And if you are PERFECT your first several games, but look like a slob doing it, all they'll remember is that you were a slob.
6) Watch the play for a good 3-4 seconds after the play is over. Don't focus too much on getting ready for the next play - there is usually plenty of time for that, and you might miss something extracurricular after the play.
7) Work with your partners, not against them.
8) Did I say see it before you call it?
9) Don't blow the whistle unnecessarily, or too early, or if you aren't sure the play is dead. If you can't see the ball, it might not be where you think it is. Remember - the play kills the play. Your whistle (except when you decide forward progress has been halted, or when the ballcarrier steps out of bounds and no one but you saw it) shouldn't be the thing that kills the play.
10) Don't mention to the coach that this is your first game. He'll eat you alive.