Sing along with recordings and try to sound EXACTLY like them- cup your hands over your mouth to direct the sounds back towards your ears. I did that way back in junior high and at first so no one else in thehouse would HEAR me, as I tried to emulate Simon and Garfunkle, and couldn't sing AT ALL.
But keep at it- I did through the years- singing along to my recordings has always been a pleasure- and I got fairly acceptable and learned a lot of things-
pay especial attention to the breathing- if you are following along with someone on CD, you'll see where they are taking their breaths- Joihn Denver and Karen Carpenter were especially good at this in my own time, and it was like a challenge to hold the notes for as long as they did- I was a bike racer, so I figured I could.
When they take a big breath, you have to too, and then not expend it any faster than they to hold the sustain on the note or phrase until they take another breath. Try not to breathe anymore than the singer you are singing along to. Karen's sing-song style is really nice to develope some range and breath. I'm playing her and singing as I write this.
DON"T REACH for it too much- this is the classic frequent mistake of most people, once they start getting over the shyness and uptightedness at the beginning.
Most people think they have to belt it out there for people to hear- when the vocalists themselves are often miked super-well and NOT really trying to extend their voice unnaturally- it takes all the warmth out of a performance and throws you off key if you try to sing too loudly.
here I am, doing that very thing-
I hadn't tried to sing over the keyboard much before, and wanted my voice heard by the camera ten feet away-
So I'm "reaching" and trying to sing too loudly so it throws me off key- I didn't care- I didn't make too many mistakes playing. I don't like going over and over things too much, so once I get something- often not perfect- I just post and move on- SORRY!
http://www.youtube.com/watch?v=ctdJz4NjuXg
and that's another factor- playing an instrument while singing adds another degree of difficulty-
if I sit down and sing this song softly into a nearby mike- I can sound much much better than here. And you'll see that too, if you focus on it- don't try to sing like youre on a big stage- that's a special talent and it takes a very healthy physique-
Sing like your'e singing into a sensitive mike.
Also pay attention to the resonance- vibrato- of the voice- it's what really separates the pros from the rest of us. You may hear yourself matching the notes and pitch, while your own voice is still one dimensional- while theirs has a "warble" to it- and adding that warble only makes it more difficult to match their breathing.