I think most things have been covered, but on holding the flag:
1. For offsides, goal kicks and corners signal with your arm closest to the goal line.
2. Make sure when you signal for fouls and throw ins, that you have the flag in the correct hand before you signal. It's a small thing, but it just looks a bit untidy when you see ARs flagging for a foul and then switching to the correct hand over their head etc.
3. As above, if for example you signal for a defensive throw and the referee over rules you and says it is an attacking throw lower the flag, change hands then signal.
Other than that, don't be afraid to tell the referee and other AR that it is your first game, and don't be afraid to ask for pointers/tips/feedback at half time and after the game.
We all have to start somewhere and no one will mind helping you out.