SPEED CONTROL OF DC MOTOR USING PWM

Hi guys. I am very new to programming and I need help very badly. I am going to use an Arduino deumelinove controller. Pleese help
I have a 24Vdc 30Amp continuous dc motor. I am getting the H-bridge made to accomodate for bi-directional control of this motor. It is a geared motor, therefore I need the torque but not the speed. I would therefore need to decrease the average voltage that the motor receives via the PWM signal. The H-bridge has pwm i/p and would cater for directional control. The motor is going to be controlled using a Radio controller. The position of the motor would be controlled using a optical encoder.

Can anyone help me with the programming and commands that I need to use to achieve radio control of the motor.
Also if I wanted to, how would I hook up a pot to control the speed so that I can find an ideal speed.
ANY HELP WOULD BE VERY MUCH APPRECIATED.

Thanks

Regards

PS. The speed is going to be constant, so I just need to decrease the speed to a lower value.