After getting a good degree in Machine Design, Spending a few years in a machine shop--including load testing-- and after a few years selling various types of Gearmotors/gearboxes
Evaluate your needs. Check the speed, torque and power involved. Take in count your boundary conditions like environment, maintenance needs, reliability, price. Check in the market all the harware that exists and can be bought. Choose the most probable transmission device to be used, and then loose sometime with numbers.
I cannot be more specific without knowing what you wanna do.