Here's one thing you can try to learn how to breath from the diaphragm.

Lie down on your back (the wonderful thing about the internet is that you can say that without it sounding like an indecent proposition.)

You see, we're all born knowing how to breathe correctly. A newborn baby has no trouble making itself heard in the next room. Over time we learn the "wrong" way to breathe, and it becomes a habit - but there is still one position in which everyone breathes correctly, and that's when we lie on our backs.

So lie on your back, and just breath deeply in and out for a while. Pay attention to how your belly is moving, and what it feels like.

After a while, take a deep breath in, and then sing a note as you breathe out. Don't try for volume, you're just trying to experience what it feels like to create a sound from your diaphragm. Do this for a while.

Finally, stand up and try singing the same way, letting a sound come out when you breathe out, and trying to have the same feeling of your belly moving in and out, and the air coming from it.

I hope this helps

Jacob