dubesor

From Sheldon Brown's article: "The cheapest way to convert a multi-speed bicycle into a fixed gear is to use the original rear hub, assuming that it is made for a conventional threaded freewheel. A fixed sprocket will thread right on, but there is no provision for a left threaded lock ring. If you go this route, it is a good idea to use LocTite or a similar thread adhesive. You can use an old lock ring from a British-threaded bottom bracket as an additional safety measure, it is the same thread. Although you can just screw on the sprocket and put everything together, the chain line will probably be incorrect."

So I just buy a new sprocket and a lockring? Or more stuff is needed? How do I know what sizes to fit my wheel/frame?

shredsled87

To be honest. I would not run that setup. If you can. I would get a rear wheel with a hub that has threads for a cog and a lockring.

HandsomeRyan

I agree with the previous poster; if you are new to this it's better to just buy a new wheelset for $100 from BI.

If you insist on using your current wheel you need a fixed gear cog; a bottom bracket lockring (not the same thing as a FG lockring); and you'll need to re-dish your wheel.