Originally Posted by seau grateau

Also: fixed cogs typically come in two sizes--1/8 and 3/32. 1/8 is track standard and may not work with the chain you have on the bike now, because the teeth themselves are thicker. 3/32 should work fine. You will also need a chain-breaker (or somebody who owns one) to adjust your chain length.

edit- whoops, beat me to it.