If your rear wheel has a freehub (as opposed to a freewheel) Surly makes a single-cog cassette that just slides on and locks down like a gear cluster. If you have a freewheel, there are single-speed freewheels available from Shimano and Dicta (to name two brands). You'll need a freewheel tool, a vise and possibly a chainwhip to get the old one off; bear this in mind as opposed to simply hiring it done by the bike shop.
If you really want to do it right, you'll re-dish the rear wheel so it's more nearly symmetrical, and that's when the fun really starts. There's something very Zen about wheelwork. There are loads of tutorials available online; I particularly like the ones available at Instructables.com