3 answers:

1. Retro Grouch's quick and dirty answer: Look at where the shift cable enters your rear derailleur. See a barrel adjuster there? Stand behind the bike and turn it counter-clockwise 1/4 turn. My bet is that'll fix it.
2. The engineer's answer: Look up rear derailleur adjustment on the Park tool website. They'll have a good set of step-by-stem instructions. Start at the beginning and don't skip any steps.
3. The lawyer's answer: Take it to a bike shop. If they can't fix it to your satisfaction at least you'll have somebody else that you can blame.