It's pretty easy, really. Yield for everyone who is coming from your right when you are approaching it. When you're inside the roundabout, everyone else who is not yet in the roundabout must yield for you. Oh, and roundabouts can have lanes (sort of) too, so don't change lanes excessively while in it. And you also don't need to yield for someone on the inner lane if you're going to use the outer lane.

At least that's how it is where I live!