It could be argued that one speed mountain bike racing began with the first Tour De France, that all those crusty refugees from prisons, wars and farm life around World War One were our spiritual forefathers in the gearless offroad arena of today...... .........which would be in part true. They rode one speeds. They had to fix their own bikes. They rode on mostly unpaved surfaces. They were by and large, looked down on by many factions of society at the time, given the proclivity of many early tour riders to steal from orchards and farms as they ate and pedaled their way across France. The modern one speed racer, Nor Cal style, rides only one gear, has to fix his or her own bike (but not as often as our overgeared peers), does it in the dirt, and is generally looked down on by gearweenies and race promoters. Why ride a single speed? You may wonder why anyone would want to  ride a single speed mountain bike. First of all it will improve your leg spin, With no gears to worry about you can concentrate on spinning your legs. Secondly, you won't have to worry about drivetrain maintenance or that annoying chain slap. Most importantly you will push your body to new limits you would never achieve pushing that sissy-ass little 20 tooth granny gear up the hills. At first you may have to push a little bit up some of the hills, but with perseverance and determination (and a little pain) you will become a better rider than ever before. Common gearing on a single speed bike is a 2 to 1 ratio. For example a 36x18, 34x17, or 32x16 gear combination would be typical gearing. You could start with a lower ratio if you live in extremely mountainous area or are new to the one speed thing.