If you don't mind spending a little money you could just buy a wheelset and put that on there. When I wanted to try fixed the first thing I did was strip the deraileur and buy an alexrims wheelset. Some people on this forum don't like this company but it is perfect for a newbie just looking to wet his beak. Anyway the rear wheel will run you about 100$ and comes with a flip flop hub (yes! you can keep that freewheel you payed for!). You may also need a new chain. I ran into this problem as my old chain was too thin, I bought a BMX chain for strength.