http://sheldonbrown.com/fixed-conversion.html
http://sheldonbrown.com/deakins/how-...onversion.html
The most important thing is those horizontal dropouts. Unless you want to spend big bucks on a workaround (like an EBB), your frame needs them.
A couple of LBS's will tell you that you can't use your original cranks and ring because they're the wrong pitch -geared bikes mostly use 3/32" chain, while singlespeed, fixed gear and BMX 'typically' use 1/8" chain- but there are plenty of suppliers about that will sell you a 3/32" fixed cog.