I am planning on controlling a motor which requires 12v 10a, i have the power supply and motor however need to control the speed of the motor.
What component would i need and what rating.
Vary the average DC voltage to the motor. That requires an adjustable (linear) power supply, or Pulse Width Modulation, where the power is turned on/turned off at a high rate (too high for the motor to follow it), but where the average value controls the motor speed.
You can add a PWM switch quite easily. That will use the existing power supply.
A MOSFET is easiest to use and you need a freewheel diode. A large capacitor on the supply is a good idea. The motor inductance will keep the current nearly constant.