Arduino Beginner - Controlling Speed of 12V DC Motor w/ Arduino (wall power)

Hi, new to this forum and just getting involved with Arduino’s. I have a 12V DC Motor (3A max I believe)… here is the link:

http://www.ebay.com/itm/12VDC-260RPM-High-Speed-DC-Worm-Geared-Motor-with-Gear-Reducer-High-Torque-/321620410856

I want to provide power to the motor via wall power and control the speed of the motor with an Arduino chip that is also powered with the wall power. I’ve done some research, but cannot find an answer to this specific question… and my prior knowledge of electrical engineering and electronics is not great (I’m a mechanical engineering student).

The motor only needs to rotate 1 direction and also only needs to rotate at 3-4 specific speeds.

What components do I need to provide power to and control the speed of the motor?

Thanks for the help!