This is a kit that actually has what you need:

http://www.nashbar.com/profile.cfm?c...All%20Products

It's got a tensioner which will take up what slack there is in your chain once you shorten it, some cogs and spacers to replace the cassette on your rear wheel, and that is all you need.

To install this stuff, you will need some allen keys, a chain tool, a cassette lockring tool, and a chainwhip. Use the chainwhip and the lockring tool to take off the cassette lockring, and slide off the cassette. The park tools website has a great step-by-step instructional thing that tells how to do a lot of stuff like this, I highly recommend it. Then you put the spacers and a cog on the hub where the cassette was. Take off all your shifters and derailleurs. Shorten your chain with the chain tool until it's as tight as it can get around the chainring and cog you want. Then install the tensioner to take up what slack is left. Then you can ride it.