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?