For a quick an easy fix, head over to HowStuffWorks:
Force, Power, Torque, Energy
How Gears Work
Convert Engine Torque to Horsepower
Of course this is no substitution to picking up a Physics book and learning the material. You basically have to grasp a few basic concepts: Force, Torque, Power, Gears, Gravitation, Friction and Conservation of Energy. Once you've grasped those concepts, you should be about set for the mechanical aspects of robots.
Boston Gear has one of the best, condensed manuals on
Gear Theory. It's very large so I'd recommend downloading it (right-click > save as).