I had the same problem. But what idid was breath deeply. And not look at the people in front of me. I did my best and u didnt win but i felt like a winner because i faced my stage fright and it wasnt even that bad. And what everyone told me was "if you wouldnt have done it , it wouldve just made you look dumber" and everyone was proud of me. Remember You Only Live Once. So live it up. Do u think the people remember you if you make a mistake in a week or two? No. Youre just lucky you get to do what you get to do and they dont. Be proud.
Start by practicing your part until you know it by heart. Then, recite the words or sing the song you'll be singing in front of a mirror to help in gaining confidence. Additionally, concentrate on what you and not what you think people want to hear or see. See www.wikihow.com/Overcome-Stage-Fright for more on this.