Set your Camera to Shutter Priority Mode - this wil alter the Apeture to compensate for changing light as you pan.
Set shutter Speed to between 1/120 and 1/60 or if your feeling lucky 1/40.
I usually use ISO of about 400.
Workout how/if your camera has a 3D Tracking AF mode.
Then just practise mate try get a smooth motion and follow the car.
Here's one of mine from last year!
Exif data
Camera Nikon D300S
Exposure 0.025 sec (1/40)
Aperture f/25.0
Focal Length 70 mm
Focal Length 71.3 mm
ISO Speed 400