So You Want to Ride a Fixed Gear

If you want to set up a "fixed gear" you must use a "track" cog. Freewheels are called freewheels because you can freewheel or coast with them. Fixed gears are called fixed gears because they are fixed to the hub. If the hub turns, the cog turns - you cannot coast but must always pedal if the bike is moving. A single speed freewheel is just that - a freewheel.
The reason I quoted track cog is because you probably want to get a fixed cog that is matched to a road chain. A track cog has wider teeth than is used on road cogs therefore a different chain is necessary. It's OK to use road chainwheels with a track chain though, that shouldn't cause any problems. The pitch is the same but the width between the sides of the links is greater.
You definitely do not want to use your rear derailleur with a fixed gear. It will cause all kinds of problems and will probably cause your chain to jump. The reason is if you stop pedaling or pedal too slow for your speed, the rear cog will still be moving chain but your chainwheel will be getting behind. This causes a surplus of chain on the top. As soon as the derailleur has taken in all the chain it can, the cranks will be forced to turn faster to keep up with the rear cog. This is where all hell breaks loose.
Since you can't use a derailleur to keep chain tension correct, you must size your chain and MUST have horizontal dropouts so that you can fine tune the tension by moving the rear wheel fore and aft. If you have micro adjusters on your dropouts (the little screws used for quik alignment of the rear wheel) remove them.
As far as what gear to use, I recommend a 63" gear but a 60" or 67" gear will work fine too. Assuming 27" (700c) wheels:

60" gears 40x18 42x19

62" gears 39x17

63" gears 40x17 42x18

66" gears 39x16

67" gears 40x16 42x17

Fixed track cogs will be a lot easier to find than fixed road cogs. I would try local pro bike shops or mail order shops that specialize in track racing. Another alternative is to rape the cog off of an old stationary bike. There tends to be one sitting in a corner of every home in America gathering dust. At least this way, part of it will get used (use the chain too, especially if it has a track cog).
Happy Cycling!