If you are still in school I would highly suggest you take a theater arts class or speach class. They will help you with performing/speaking in front of people. Since the semester (or quater) probably just started you should be able to. Unless the school dumped those typ of clases. If you are already out of high school then check with a local community collage.

If that is not an option then I can only tell ya what I do. I just get into a "zone" if I need to perfom infront of others that I do not know and not worry about what they think.

Try and focus on your playing and forget the audience. Yeah you should make eye contact and try and have a good stage presence but you might want to forgo those since this is your first gig but it would also depend on what type of gig.

MSF