Lowcurtz,
I recommend to you, to spend some time looking at David Cook website which is suggested by blueroomelectronics.
Then, take your time and build a plan about all of your project. For example
Mechanical part
a) Build Chassis of the R/C car or buy a ready mad Chassis
b) How the motor will fit in the car if you building a Chassis
c) where You will place the Ultrasonic sensor
Electrical part
a) what kind of motor control I will be using to interface with the PIC and Hbridge
b) What kind of interface will be done between the PIC and the Ultrasonic sensor
Programming part
a) How I will program the motor motion
This is just a start, Try building a plan that you will follow and ask questions specifically to the problems you have. Specific question.
GL