I'd pick something you want to do or understand that clearly requires some sort of math, and figure out what you'd have to learn to do that. Math is enough of a grab-bag of fairly different things, especially at the intro level, that I find it easier to wander through with some sort of direction--- so I have some reason to want to learn this particular bit of math, as opposed to randomly choosing subjects (i.e. should I learn some combinatorics, or some differential equations?).