I've got two motors which I can use to rotate a wheel. However, these are 6 volt, and the Arduino only outputs 5 volt. I checked the resistance and it's approximately 500 Ohm.
I tried driving them but nothing happened. How do I change the voltage to 6 volt? I do realize that it won't be as powerful.
Driving things that use more current than 40mA on a single pin is not good for the arduino, check, how much current it takes 
(The pins can be damaged)
That's why the thing motorshield was invented
The best way to do this is to get a npn transistor and connect the middle pin to a pwm pin on the arduino, powersorce and motor to the other legs of the transistor
a diode across the motor will help
here is a site that helps:
http://todbot.com/blog/bionicarduino/
more specific on the motor control:
http://todbot.com.s3.amazonaws.com/bionicarduino/bionic_arduino_class3.pdf
Read this pdf from page 6 and onwards (powerpoint slide size so it won't take long)
David