If you have a fixed/free hub (as 79pmooney described) it's better to run a singlespeed freewheel on the free side as it has more threads on it, but you can also put a singlespeed freeweel on a fixed side because, even though it has less threads, you actually never generate more force than you would on a fixed cog and the number of engaged threads is fine.
Make sure you have front and rear brakes with practical brake levers if you're going from fixed to singlespeed.