Singlespeed MTBing works best on tightly winding trails. You'll need a gear ratio that is much lower than you would use on the road. Winding trails are best because endlessly spinning a tiny gear through long straight flat sections of trail is just no fun.

If you ride several trails, you'll probably need several cogs or freewheels. With a 34 tooth chainring and 26" wheel, I use a 15, 16, 17, 18, and 20 tooth freewheels depending on the trail. I'd start with something close to a 2/1 ratio and adjust as needed.

Prepare to spend a lot of time out of the saddle mashing up hills! This is a good thing...I find myself blowing past geared riders on climbs. As a rule of thumb, I generally use the highest gear that I can successfully clear the steepest hill..that way I can go fast on the flats but still not have to walk up the hills. If its a really tight, winding trail, where high speeds are not possible I will gear down even more.