Flip polarity on Darlington Tip120
Sounds like you are looking for information on an H-Bridge. The tip120 is an NPN transistor. H-Bridge uses 4 transistors. You'll need two NPN and two PNP transistors to create an H-Bridge capable of multi-directional motor control.

An H-Bridge works by allowing directional control of a DC motor by turning on two of the transistors at a time. 

The choice of the two transistors decides which way current flows, dictating the direction of the motor.

Great tutorial here: http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge 

Here's a tutorial using the tip120 
http://www.instructables.com/id/Use-Arduino-with-TIP120-transistor-to-control-moto