Do you know the referee?
Just listen carefully to his or her instructions. Remember, you are also a referee, just with a flag, instead of a whistle.
Personally, i find assisting harder than refereeing, you need to keep up with the second last defender, as well as watching the rest of the game. Saying that though, i think i prefer assisting.
Anywho, relax, listen carefully to the referees instruction, maintain plenty of eye contact with the referee, this will help avoid you both signalling differently.
As with refereeing, be confident and smile, the referee will help you through it. Don't be afraid to ask questions if you are unsure about anything
There are some posts on here, with instructions for assistants, have a read of some of those, they may give you an idea of what you can expect