you need two things: a driver circuit to flip the polarity of the motor connections ( to change the direction) and something to tell the motor what speed to go at.
The standard way to drive a motor in reverse and forward is called an "H bridge". There is a simple onehere.
It will actually drive two motors in both directions. A Dout pin will tell the motor which way to go.
The same board takes an input pulse to determine the speed. this is a digital PWM pulse that is generated in the Arduino-- you would not want to use an Aout pin. If you read up a bit on PWM motor control, you can then easily modify the RC servo code for the arduino to drive your motor at variable speeds.