This video tutorial shows how to control a DC gearmotor with an Arduino using a transistor! The video covers the items used, how to setup the pins and wires and a link to a few related videos with additional detail:
Six Ways to control a DC Motor: http://www.youtube.com/watch?v=BiJMsMguv-M
Controlling a DC Motor with a Raspberry Pi & Transistor: http://www.youtube.com/watch?v=_iLbGR7IG8Q
Transistor from Adafruit: https://www.adafruit.com/products/976