Simply stated, the muscles within the small air passages of the lungs go into spasm and narrow the airways, causing wheezing and breathing difficulties. A doctor should monitor your condition and, if necessary, prescribe appropriate medications.
Asthmatics can do a number of things to help themselves, though.
Figure out what triggers your asthma, and eliminate allergens or irritants at home and at work. Make a special effort to keep your bedroom allergen-free. Sleep with a synthetic pillow, not a feather one. Don't smoke, and avoid exposure to air pollutants. Wear a scarf around your mouth and nose when walking or exercising in cold winter air to warm the air as you breath it in.