Originally Posted by JohnDThompson

If it's a single freewheel (as opposed to a fixed sprocket) you'll need the tool to remove the freewheel:


If it's a fixed sprocket, the procedure described by @TejanoTrackie above in Converting a fixed gear to a single speed is what you need.

Single freewheels can run anywhere from ~$10US to over $100US for a fancy one. I recommend avoiding the cheapest ones. Shimano single freewheels are reliable and affordable; I've had good luck with them.
Individual fixed sprockets come in a similar range of price points. Again, avoid the cheapest ones; they tend to be stamped from plate and have poorly formed threads which can damage your hub. You can expect to pay $20-$50US for a decent quality machined (not stamped) sprocket from Shimano, EAI, Miche, etc.