There are two general approaches to modelling a car:
You can go the "box-model" route where you start with one cube shape and you extrude/cut/etc. until it looks like a car or build it up face by face. The former method yields quick results, but it is mostly suitable for blockier, plainer cars, such as sedans from 1960 to 1999 or large commercial vehicles with plenty of flat panes. The latter approach is more suitable for curvier and complex vehicles, but sometimes it can get rather confusing as you may get lost in the little details.
Nowadays I'm building up my cars face by face with the use of extrude, knife, subdivide etc. tools. I start with the wheel arches, as they are the most annoying parts to make. After that I build up the wings (fenders), the rocker panel, the boot (trunk) and the bonnet (hood). Then I make the roof and bevel the A and the C or D pillars (depending on the car). Afterwards I cut out the window openings.
If you want to add smaller details, you can always start with either using a different cube (good for things like door handles, etc.), or by duplicating one part and cutting/extruding what's needed (eg.: decorative chrome strips on the side).
If this sounds confusing then I might perhaps elaborate this in a separate thread with pretty pictures and stuff. Can't promise anything, though.
