Ah, circles are hard. >.< After some experimenting, the way I do it I take the circle tool (not really sure what it's called, but it makes an outline of a perfect circle for you. It's icon looks like a circle.), put to where I want to the circle to be at, fill it it in completely, then go back and make another circle, only a little bit smaller, put it inside the bigger circle, and then erase everything that was in the smaller circle.
Hope that made any sence. ^^U