Use this instead. More efficient:
**broken link removed**
Instead of burning extra energy in the potentiometer to make the motor run slower, it uses the potentiometer to vary the resistance that the capacitor charges through which controls energy pulses (for simplicity) to the motor to control the speed. This way you don't burn off extra energy so it's more efficient and things can be smaller and simpler because they don't get as hot.
THe schematic in the link shows you what each of the 3 prongs of a pot stand for- the ends of the voltage divider, and the wiper that varies along the middle to change the ratio.