OK, so you are ready to turn your bike into a single speed, now what?
Well you can go online to one of many sites: Sheldon Brown being one of the best! Single Speed Outlaw is not bad either. Unfortunately both of them are apparently afraid of pictures, so we can recommend Google Image Search.
So essentially if you don't have a dedicated Single Speed bike, your biggest problem is chain tension. If you want to benefit most from the single speed fun, you can take your shifters, cables, & derailleurs off and even your small & big chainrings if you have bolts/spacers that will work for the one remaining chainring, but your chain still won't stay where you want it.
Your best quick bet is then to find the gear you want on your cassette (something in the middle). Then we recommend using an old (or your normal) derailleur, with a small piece of a spoke and nipple replacing the cable or a dedicated true chain tensioner. Take as many links out of the chain as possible still going through your tensioner, and voila you have a Single Speed.
If all else fails, you can just put your bike in a gear you like and wrap about half a roll of duct tape around the shifters making them no longer functional!
Just for a frame of reference, I have recently been riding in the area of the course on a 29er with a 38:20 (55.3 gear inches) gearing, which is about the same as a 32:15 or 34:16 combination on a 26" mountain bike. It's tough on some of the climbs, but fast on everything else.
Good Luck!
Give us a shout if you need help.
-the management