Shift into a gear combination that makes your derailleur arm point straight down. Now look at the derailleur from the back. If it looks like it's pointing toward the wheel (common), that's bad. It should point straight down from that angle too. On a 6-speed I can usually "eyeball" it into alignment. Keep the derailleur in place and clamp a big crescent wrench over the derailleur to bend the hanger back straight. Sometimes I can bend it with just my hands.

Don't be tempted to remove the derailleur before bending the hanger because you might ovalize the derailleur mounting hole. I don't know a cheap, easy way to fix that.

Good luck.