It could be a problem with the bridge or the neck. It's hard to say without actually seeing the guitar.

Given that, I would suggest taking it to a guitar shop and getting it fixed. Unless you really aspire to be able to repair guitars, and with that I mean having one (or a couple) around you can afford to mess with; I would put it in the hands of a professional to repair for you.

Guitar Center, or a local music shop should be able to adjust it for you. Good luck. :cool: