http://www.3dluvr.com/content/article/84
This one seems to be a very good one, not too difficult. Check it :wink:
It starts with a box but you can start with a polyline as I've done myself.
Then it's all about extruding edges and putting them in a smart way.
Then when the whole shape of the car is done, start to modelize the details.
It's a good one.