Smile. It will put you and the audience at ease. Few people can resist this friendly gesture when it is sincere. You can build allies this way before your performance even begins.
Move. Standing glued to one spot will make you seem uncomfortable. No matter how goofy or spastic you may feel, it's better than looking frightened and amateurish.
Take on an alter ego. Give this persona all the characteristics your rock star self should have. Improve your perceived flaws by ignoring them altogether. You may be afraid, but your persona can be fearless. Give it a name. Beyonce calls her alter ego Sasha.
Make eye contact. You have to connect with the crowd. Choose a few friendly faces to focus on. You may want to ask friends or family to attend your first few shows for this purpose.
Cover your mistakes. To develop confidence for stage performance, you have to know you can recover from anything. Minimize your mistakes as best you can. The audience is likely not to even notice if you don't overreact.
Practice. Your confidence for stage performance will get stronger with experience. You'll understand what to expect and become more adept at handling surprises.