I have one string that buzzes on my guitar. Its in tune, and it sounds alright, but I cant keep it from buzzing. Its the only string that does it. Any suggestions?

yea theres a lot u can do.

if its an electric, what kind is it?
1.u can raise the action, lifting strings away from the fret board. until the buzz is gone.

2. check the trussrod, see if it needs adjusting, if thats the case u might want to bring in a pro.

3 also a change in string gauge that can stop some minor buzzing.

4. is there a fret thats up too high, a local guy can use a tool to pound it in a little.

www.icepoint.com/guitar/Setting%20up%20your%20electric%20guitar/index.htm

^link shows how to "set up" a guitar the right way but, if ur not sure what ur doing, go to a local store and buy a new set of strings from the guy. while ur watching him put them on, mention the buzz and see if he can tell u why its doing that. it might just take a couple of minutes to fix. maybe just the cost of the strings.
good luck

Do what Dave said and also, if you have never had your guitar set up by a pro, I highly recommend doing that; It`s well worth the cost.