So I just bought Rock Band for the PS2. I know, I'm awesome.
I'm pretty good at guitar/bass, and I'm playing medium on drums.
Now, I'm in desperate need of some tips for vocals.

See, I'm not a singer. I have no natural talent for singing. (Although my dad thinks otherwise.) My friends know I can't sing. When I told all of them I got the game, they all said, "I'd like to see you sing."

My goal is not to sound pitiful when they come over. I'm getting better at hitting the notes, but I'm having a few issues. For one, I have a rather low range for a teenage girl, and it's not a very broad range, either. Plus I'm using my throat muscles to sing. I've been told to sing using the diaphragm, but I don't know HOW to do that.

Any tips?