School Musical Audition tips?

My middle school is putting on school house rock jr. and I'm sooo scared for auditions!!! I'm always acting, signing and being the cl clown, so everyone expects me to do really well! Unfortunately, we will be auditioning with something that we will learn once we get there!! I believe that I really am a good signer and actor, but i have to perform in front of everyone! My voice gets very odd when people ask me to sing and i really haven't sung in front of people... Do you have any tips for calming my nerves, preparing me and singing to the best of my ability? thankkk you!!!