How to sing with your diaphragm?

i have been sing for a wile and am noticing i am getting a bit of tightness in my throat when i sing.it hurts a little bit (like im straining it) i got told i should sing from my diaphragm not my throat but every time i try i cant do it. i have been on all the wikis and websites but cant find anything that helps me sing from my diaphragm. plz help

try practicing when you brethe in for your stomach to go out and when you brethe out your stomach to go in. practice, practice ;D
i sing too and play clarinet!