Chosing a martial art?

I am planning on taking a martial art, and devoting time to it. I have no experience whatsoever in any kind of martial art. I have never really watched any competitions or anybody practicing any martial arts of any kind. I am fairly flexible and strong, and am fairly tall. I am not exactly on the popular end at high school, and am often picked on, but I have only been in a couple of real "fights". When these happen, they are usually broken up, and it is hard to tell who "won". Besides this, I am pushed around a lot and I try to avoid fights. I would like to find a martial art that will help me not only to fight back, but , if I needed to, instigate a fight. I really don't care what everyone out there has to say about fighting, but what kind of marrtial art would help me quickly win fights, and improve flexibility and coordination?