When you buy a new radio, it generally comes with a receiver as well and sometimes a couple servos. If the car still has a receiver in it, you'll have to unplug the servos and plug them in to the new receiver.

All that's left to do then is to adjust all the settings on the radio to run the car properly. If you totally new to this hobby, I would suggest hitting up your local hobby shop and have them help you pick out a proper radio and help you set it up for your car.