How do i get over stage fright when i perform in front of the school?

Next month, we have our 8th grade banquet where there will probably be about 100 people, more or less. My friend and I are planning on performing, she will play piano and I'll be playing acoustic guitar and singing. I have absolutely no problem with guitar and singing the song, but I've NEVER sang for people before (other than just singing for fun with friends). I'm afraid that I might screw up because of that. How can I prevent that from happening?