Well if you are looking to learn just one, or can only afford ONE I recommend BJJ it will work as great self defense and anybody who doesn't train BJJ won't know what to do when you take them down.

There are plenty of free if not very cheap boxing gyms you can try on the side to well round yourself

taekwondo is the best martial arts

One where you get a good impression from the main instructor, and close enough that you'll actually go routinely.

Fighers.