Chosen, I don't want to put you off, but you can't seriously think that somebody can teach you something about fighting (even slightly useful) in theory.
My first advice would be to start building up your muscle mass (just lift some light weights two times a day, do some push-ups in series and start having proper nutrition, a.k.a. get yourself in shape). Furthermore, have some lessons in martial-arts. I would advise you to forget about kick-boxing, thai-boxing or such, since you say you can't lift much and I suppose you are not in shape for that kind of fight (you would probably just have your behind kicked in some sparing and you'll be fed up with it in very short period of time). Instead, take lessons in some other stand-up martial art which doesn't require that kind of power and body structure. My bet is on karate in that case, as it is an art which trains your reflexes and speed, thus teaching you how to use your power in the best possible way.
Some time later you can get a punching bag so you can train at home as well (let's say 20-30 minutes a day). Your punches and kicks will become stronger and more precise with time, and when that combines with good reflexes, speed and proper technique, it can become quite a combination. I'm pretty certain this would fulfill your needs!