This is an excellent way to get started quickly with robotics!
You may have read many websites about servos, gear head motors, DC motors, stepper motors, and the pit-falls associated with each.
We swear by these gearboxes because of their price, their ease in assembly, and the ability to be controlled by low-cost motor driver ICs.

This gear box can drive two wheels independently in either forward or reverse.
The twin-motor gearbox can also be configured for two different gear ratios (fast and faster).

Gearbox comes unassembled as shown.

• Includes gears, housing, motors, lube, and allen wrench
• 58:1 and 203:1 gear ratios possible
• Motors operate from 1.5-3V
• Can be controlled by L293DNE, SN754410, and other low-cost motor drivers
• Mounting holes and hardware included