You need something to boost the current capability of the potentiometer. There are several options, many of which are nicely presented and described at this page:
http://www.pcsilencioso.com/cpemma/ef.html
Those options are all "linear" regulators - a transistor conducts more or less, to adjust the output voltage. This forces the pass transistor to dissipate power, which means it needs a heatsink. Usually, power dissipation is worst when the fan is running at about half of the supply voltage. If the fan draws more than a few watts, it may be worthwhile using PWM (pulse width modulation) to avoid the heat dissipation.