Your wheel is out of true, this happens under normal conditions as spokes stretch.
You're going to need a truing stand, some lubricant, and a a spoke wrench. (I've heard you can use zip-ties attached to each side of your wheel fork, facing inward, then cut to size, but I don't think this makes for very accurate measurements.) Most bike shops I've worked with will let you borrow their stand for a few minutes. If not, find a co-op or make new friends.
Put your wheel in the truing stand and start by lubricating each spoke nipple. Once lubricated, use the guides on the stand to determine where the wheel has been skewed to one side. Select a spoke at that site, opposite the side of the skew, and tighten the spoke to pull the wheel away from the truing stand guides. Also lightly tighten spokes that are next to this initial position (but still on the same side) to even the strain on your spokes.
A word on which way to tighten the spoke - remember that you are probably going to view the spoke nipple from across the wheel axle - therefore the directions to turn for tightening is reversed from normal. Also, it is preferable to tighten spokes rather than to loosen them, as the main cause of a wheel being out of true is/are a stretched spoke(s). Even so, be aware that spokes can occasionally be over-tightened, it may be better to loosen a spoke to compensate for a wheel pulling to that side. As you tighten spokes, you will hear lots of popping/straining sounds of the metal. This is normal, and may continue for a few miles of riding after adjustment.
As you adjust areas of the wheel that are out of true, slowly tighten the guides on the truing stand to find the next area that needs adjustment. Also be aware of the differences between getting the wheel perfectly in true, as opposed to good enough in true. You can spend an hour truing a wheel, but it probably won't be much better than it was 10-15 minutes in.
Re-install you wheel on your bike and adjust your brakes as normal.