[AgentZero]

Instar, I've got almost the opposite problem to you. I'm about the same height (ok, an inch taller), but you've got about 60-80 pounds on me, and I wouldn't exactly say a sizeable portion of my mass is muscle either. However, a not-so-pleasant childhood has left me extremely proficient at, as you put it, beating the tar out of punks.
I've gone along to a few martial art classes, but found that their main purpose was to give people the confidence to be in a real fight, rather than actually teach them anything that they could really use. If you don't feel you have that confidence, any martial art will give it to you. If you feel you have it already (and I don't see why someone your size wouldn't), I can tell you about 6 simple tricks that'll bring you out on top in just about any fight.
Which I'll post later cause it's lunch time and I'm starving. ;-)