Arduino – Control a DC motor with TIP120, potentiometer and multiple power suppl

I would read this as I think it might be more apropos. http://itp.nyu.edu/physcomp/Labs/DCMotorControl

I have made this circuit, the problem is speed of motor can not be controlled and also i want to drive motor in both direction.
The code has to be modified to drive in both direction but how?

Regards,
Shubham