How can I become more outgoing with more self-esteem?

Hi everyone… I am so nervous because I am going to be in High School theatre this year! And I was already last year when I was a sophmore but I was so nervous, I cried at home after play practice because in a few days we would be doing our actual play at another school in front of judges… I always feel nervous talking to someone I don’t know or in front of people I don’t know , but when I’m with family and friends I am like really talkative and I make my friends laugh all the time, but I just am self concious I guess of how I look and seem to other people… (mainly other classmates) My theatre teacher is awesome! She knows I am insecure because she approached me asking why I was… She said to me: “You are self concious aren’t you? But I don’t know why you would be you a are very pretty and nice girl.” I felt embarrassed but happy at the same time, but I was still nervous about the play that I almost quit and let down my theatre class mates! I felt so horrible… but the thing is I love acting and that kind of stuff, I just get REALLY REALLY NERVOUS. Well my question is: How can I become more outgoing toward people and how can I have more self-esteem for myself?! Thanks and please help me out here! I have been stressing over this for the entire summer!!!!! And thats no fun! Thanks for your time!