Caffeine before a performance is bad. Your metabolism will already be racing with adrenaline once you're on stage. You'll get the shakes bad.
I usually do a swig or two from my flask before I go on stage. But you're underage, so don't do that.
On a more serious note, there are scientific studies that support the notion that being nervous makes you do better, provided you acknowledge that fact as well. You have to accept that you will be nervous, but you also have to believe that the nervous energy, the adrenaline that floods your veins will help you focus and think more clearly. So being nervous is good! If you weren't nervous, I'd say you have mental issues.