Go to Sheldon Brown's website and look for information. You could also look at the opening page and read the other ten threads that are currently going on about this same topic or use the search function and read through hundreds of threads that deal with this exact same topic. You could also type it into google and read through millions of hits on converting a bike to a fixed gear/single speed. You could also buy a track hub, spin a freewheel on, shorten the chain, possibly remove 1 or 2 front rings and you are on you way. You could also take the wheel off, have the freewheel removed, spin a bmx single freewheel on, remove 1 or 2 chainrings, shorten the chain and be on your way.