You could write pages on preparing a car ready to paint but here is a very brief guide:

Filler can be blacked down with 40 grit then 80 and finished in 100 grit.
Panel prep I usally use 120 grit and then go over it again with 240 grit on the DA. Any areas a DA wont reach a red scothbrite or 240 by hand can be used.
Primer is then blocked down with 320, then DA`ed with 320 and then finished with 500 grit.
Top coat flatting envoles 1200 wet and dry on any massive bits of shite, 1500 wet and dry on the DA then a 3000 grit Trizact pad over that, then reay to polish.

Hope this is some help.