The more training and experience you get, the more comfortable you'll be. Take acting classes outside of school and audition for community theater or join a summer theater camp. That's where you start.
Most actors have some stage anxiety - you're not alone! The goal is not to get rid of the anxiety, but to channel that energy into your performance. So don't ignore or fight the anxiety, but accept that it will happen and learn to control it.
Preparation (training, rehearsals and the like) and focus can help you do that. Work at changing your perception - people are not there to judge you, but are there so you can share your talent and passion with them. When performing your focus should be on you and your character, not the audience.
Things like stretching, deep breathing and other stress-reducing techniques can help you control the physical response to anxiety. Taking care of your health - exercise, avoid caffeine, get enough sleep are important too.