Put one hand on the top part of your chest. Now put the other hand on your belly or abdomen. Breathe in through your nose. Don't let your chest move. Can you feel your abdomen go out like a balloon?
This way of breathing in (inhaling) is using the diaphragm. (That's the part right there, just below your chest). Now do it again. Be sure the hand on your chest is still as you breathe in through the NOSE. The hand on your abdomen moves up.