How can I become a better singer?

Question:

How can I become a better singer?

I don't want to take lessons! Is it possible to teach myself to sing?? I don't need to be great, I just need to be good enough to get a part in my school production of High School Musical.
My facts:
*I was in choir from 2nd-4th grade. I hardly remember anything I learned though! I just remember my teacher saying "elaborate your words" and "sing from the diaphram"(how do you do that??)
*I think I sound okay. I definetly wouldn't call it great
*My voice doesn't go very high.
*I have trouble staying on the same note.
*The role I'm auditioning for has no solo. I sing only for a few minutes.
All help is HIGHLY appreciated. Thank you!
P.S. IF getting a voice coach is the only way you think I'll make it, please tell me! Where do I find a voice coach??

Best Answer - Chosen by Asker: I believe it would be alot easier and quicker for you to get a voice coach to help with your singing, espically if your school production is soon. You should ask your music teacher at school, they mite no of one or mayb able to help you themselves, also the phone book will have lots of different voice coaches in it. but you can teach your self to be a better singer, practice practice practice! play a song on a sterio but have it quite low so your voice would be louder than the music, practice that way, an also to help breath from your diaphram, place your hand on your stomach and feel your breath going in and out. and sing to the rythm. Hope this helps