Practice performing for other people, even if it's just friends are family who happen to be over. It's always different when you're playing a piece by yourself versus playing a piece in front of other people, and if you put yourself in the latter situation more often that you will become more used to it.This might be kind of obvious, but make sure you really, really know your piece. In all of my performance experiences, I've always felt most confident when I've known that I've practiced my piece very diligently, and that it will be easy for me to get back on track if I make any mistakes (another good thing to do is find some spots that you could jump to if you made a mistake and make sure you can easily start playing from there). On the day of the performance, make sure that you are extremely warmed up before hand and wearing relatively warm clothes. This will help prevent any stiffness or shaking that is often associated with nerves.
What are you being asked to play?