With the motor, you cant simply add resistance in line with it to limit the current because the voltage that is 'dropped' accross the pot will obviously fry it. To control the speed of a motor you would need some sort of speed controller. You could use a 555 timer circuit which uses a pot as the input to tell it how many pulses to output and then feed this into some sort of power transistor.
Have a look here, half way down the page,
http://www.educypedia.be/electronics...torcontrol.htm
loads of DC motor control stuff.
It's not a straight forward thing!