Depending on how badly you've bent it, you might as well try and fix it before throwing it away. Get yourself a spoke wrench, and with the wheel on the bike, loosen the spokes a few turns all the way around. Then, using your brake pads as guides, the idea is to pull the rim in the direction you want it to go by tightening the spokes that are on that side. Start with one turn at a time, then go to half turns, then quarter turns. Once you've pulled the largest out of round sections on both sides, start in on the smaller spots that aren't true. Most likely, you'll make the wheel at least rideable.