How am I supposed to exercise when I get out of breath within minutes?

Get physical exam. Perhaps you want to get a complete physical with your family doctor before taking up a serious exercise program -- and your doctor can rule out any problem that may relate to your breathing difficulty.
By gradually. Increasing your exercise duration, you can shed all those excess pounds and get into shape. Consult your doctor to be sure there is no contraindication to exercise, and then embark on a program that will improve your well-being.