all you should need is a new rear wheel with a flip flop hub single speed freewheel or fixed gear cog and a new chain. If you really feel obligated you can get a new chaining and chaining bolts. Some people will also say that you need a chain tensioner but if you have long dropouts (where you put the wheel) then that's really up to you. Look at some of the stickies and threads above to for more information.