Breathe through the diaphragm not through the chest. Gently place both your palms on each side of your belly (just below your belly button).
When inhaling, expand your diaphragm to the point where you can see your hands rising on your belly. Make sure your chest does not rise as well. When exhaling, suck your tummy in towards your back, and continue as to squeeze as much air out as possible. Expelling all of that carbon dioxide from your lungs to inhale again so fresh air.
Try doing this as slowly as possible. When you feel more confident with this type of breathing, count to four when inhaling and to six when exhaling.