Hi mohame. I have some experience with this. I made a board a year and a half ago to control my RC-10 using an Atmel atmega 8-L. **broken link removed**. Hopefully you are using a full fleged RC car kit and not a pre-built cheap derivative.
In an electric RC car kit there is always whats called an ESC(Electronic Speed Controller) and usually a servo to handle the steering. Both of these are controlled via a 50hz PWM signal from the radio of the RC car.
I bet your micro-controller has PWM output pins. If you have two of these, you can control the ESC and servo using the pwm output of the microcontroller. Just be careful as RC car electrical systems encompas whats called BEC(Battery elimination circuitry). The ESC usually puts 5V on all the three-wire servo and radio connectors so you need to keep that in mind when interfacing your microcontroller with the car.
Best of luck!