No worries man, this is what you need to do. First take your chain into the shop and have them tell you what size chain you have (it will either be 3/32" or 1/8"). Go ahead and buy the same size chain as you had before and talk to them about getting a chain breaker that will fit your chain. The chain will be too long, you use the chain breaker to take a section of it out in order to make it the right size. In all likelyhood the bike shop will throw the chain on for you but the chain breaker is handy to have especially on fixed gear bikes.

Oh, and as far as chains go anything by KMC is good or whatever they recommend. You should be able to get a good chain for $10 to 15.