Whichever ya want. Self defense is less about the firm and more about the technique. You're still 19,so your muscles has some time to reshape yet. You're fairly short too, so you won't be having much trouble with equilibrium and stuff. I'd recommend karate myself, because that's a martial discipline as much as a martial art, but really, as long as what you're learning it's something with a solid background (discipline, control, focus and technique) you're good to go.
No matter which you choose, it'll take time and effort, a lot, to be good at it, so choose one ya like and keep doing it if ya want to be good at it.