I think my previous experience with learning a woodwind instrument really helps my singing. Eg: knowing where to breathe, breath control, etc. As to how to sing from the diaphragm, you can try to lie down, just relax and breathe. Naturally, the tummy will go up and down. Then remember how it goes and try to do that during singing. Breathe deeply thru the mouth and push air down to the tummy. Singing time, squeeze the tummy for support.
To learn to sing high notes, just keep singing with the CD. Just like someone said earlier, need to expand the vocal range to the high pitches slowly. Sing along with Jeff Zhang. His voice is wayyyyy high.
Another thing to note for singing high pitches is NOT to start with a cold throat. Need to warm up first with mid range songs. I like to sing fast upbeat songs for warm ups. I find that after warming up, I can reach high notes easier and also sustain better.
And how not to go out of tune? Of course must know all the notes of the song well. But I find that it is important to pitch to the instruments as well. Especially singing slow songs with lots of sustain parts. Very obvious if I go out of tune. My ears need to be very active to find resonance with the guitar or the keyboard. I find that if I pitch to the instruments, the whole band would really sound as one. The vocal is fundamentally one of the instruments of the band anyway.
I'm not trained in singing. Just some of my own observations here.
