Build a Dual H-bridge PWM Controlled DC Motor Driver
Useful circuits can be as simple as a paper clip. Here is such a circuit. It can be so simple because all the complicated and hard stuff has all been integrated into a single IC by a clever semiconductor manufacturer. Plunk the chip on a little printed circuit board, add connectors, and you're in business. Motor driving business in this case. Hook it up to a DC motor and let her rip.