In this video I show how to create gears, how to make them mesh, how to put them together in a gear train, and how to make them move. Made on the occasion of me and my intern trying to figure such things out for some plywood rack and pinion gears. Good luck.
FYI I also made a Rack & Pinion Gear tutorial here: https://www.youtube.com/watch?v=khG4Zk5XZrw