riderx

tFunk -

Let's make sure you are on the right page as far as terminology goes:
Freewheel - a gear or cluster of gears with the ratcheting mechanism built in. It is threaded to mate to a threaded hub.

Freehub - a spline unit on a hub that contains the ratcheting mechanism. A cluster of gears, which is also splined, slides over this unit. The gears themselves do not have the ratcheting mechanism. Neither piece is threaded.

If you do indeed have a freewheel hub, you can screw on a FIXED COG (not a freewheel). It has the same threads as a freewheel but does not have a ratcheting mechanism. this means that when the wheel moves the pedals also move - no coasting. An English threaded BB lockring can be used to secure. Most bikes these days don't use these. You can get one at your LBS or check www.thethirdhand.com Alternately, uou could also spin on a BMX freewheel and use it as a single speed. No lockring is necessary.

If you have a cassette wheel, you can make it a single speed but not a fixed gear. Break apart the cassette, get some extra spacers, use one cog and the spacers to get things lined up properly. Step by step instructions here

Redishing? stick the wheel in the frame and you will know right away, but 99% of the time you will have to.

You need to verify that you have a freewheel hub and not a freehub and you also need to get a fixed cog instead of a singlspeed freewheel if you really want to build a fixed gear.