We all know our bikes are only as good as the tires beneath them. That being said even the best tires on the market can only perform to their potential when rolling on straight rims and whether you hit a large bump or had your cycle backed over in the driveway they can loose their roundness. Since rims, especially high-performance ones, can be so costly your first attempt should be to straighten them out instead of instantly buying replacements.
Finding If Your Bike Is "Out of True"
The first thing you need to do is flip your bike upside down and spin the affected tire (removing the brakes and calipers if necessary). The tire should spin throughout unimpeded but if you hear a click or a rub then there is some oblong-ness working. Listen real intently to which side the clanging is coming from because it's very important.
Straightening The Tire
Once you've determined which side is out of level, you're going to take a spoke wrench and tighten the opposite side of the tire. That area should be fixed but keep spinning and listening until each rub area is tightened. There will be some give and take here because for every side you tighten, you'll want to loosen the opposite side so that there isn't overkill on half the wheel.
As with many bicycle projects like adjusting gears or removing slack from a chain, straigtening a bike tire is very much a gradual process. You'll need to make adjustments, then test, then make adjustments, then test until your wheel spins perfectly (or close to it) in true form.