*Yes! Another theater person!*
Ok, yeah, I get it. I'm in my school musical and choir and everything and I worry about that every day. But really, if you do, what's the worst that can happen? If your voice cracks, no murderer is going to break into your house and stab you. You might be embarrassed for a little bit, but trust me, it'll wear off. Drink tea or water or whatever, and practice, and don't stress yourself out too much. Maybe the day before, go do something fun with your friends or something, because that sets your brain into a good mood for your show or whatever stresses you out. Break a leg!