Tips for singing in front of people?

I've never sung in front of a crowd before, but everyone says I'm a good singer. But then my brother says I'm rubbish. So I decided to sing in a performance of School of Rock that we are doing in our music class. I don't know why I did it, I think I wanted to boost my confidence and see if I could really sing. I'm singing with other people but I have my own parts. I'm scared that I will be rubbish and of what people will think of me. I could hardly sing in front of my teacher, so how will I be able to sing in front of the whole class? Has anyone got any tips on how to redue nerves and build confidence and not care abut what people think?