Well, start out with simple stuff like walk a mile or two every day, or at the very minimum once a week. Go to a local gym or PAL (police athletic league) and ask if they can help you get a routine started. Myabe start out with learning the basics (shadowboxing, footwork, etc) if you are just looking for the physical aspect, then start jumping rope, and shadowboxing/aerobics. Also, if you learn the footwork, dancing around the ring and shadowboxing will help too. Good luck.
PS I don't know much about dieting, just whats good for you, and whats bad.