Next step is to find and remove the seam lines from the car body. The car body is one piece, but is cast from a multi-part mold, so there will be little bits of flash where the mold plates met. You will usually find them along the sides on the hood and trunk, and on the corners of the bumpers. They need to be gently sanded down smooth.
Next for the body is primer and paint. Make one of
these, they help a lot. Take your time, and let the paint dry well.
In the meantime, go to town putting the suspension/chassis/interior together. Spray what you can, but brush painting won't kill you.
Then you get to polishing, and I just can't explain it better than Guiddy already has, so I refer you to:
Painting model car bodies! Full instructions!
It's all in there.

Keep us posted!