Well there are a lot of things that you need to take into consideration before you start modeling a car,
First you have to have your referance images. A GREAT place to find that would be www.suurland.com
. Aweosme bank for car blueprints.
Next you have to decide wether NURBS or POLYS would be bettter. Does the car have a flowing shape? Nurbs would be more efficient there. Or is it boxy or have sharp angles? Polygons or Subdidvisions would be more effective there.
If you decided to go to with POLYS, then are you gona modell all of the creases and stuff with actual edges? Or are you going to take it into Subdivisions and use the crease edge tool.
Then, you have to be able to UV map the car.
Then you have to texture it. Do you know what color its gona be? How to make realistic paint reflections?
Then you have to worry about rendering.
With NURBS, its kinda hard to keep it all in check so that in redering it will come out better. So that you dont see many seams and etc. Then With poly's thats not the case.
For car rendering you should check out several of the many HDRI tutorials out there on the web, and even download the free HDR shop, which I am sure you will be able to find with a simple www.google.com
As for your quesetion about what you should model first... etc/
I normally start with the front and the bonnet, and then go to the rest of the car, starting with a plane and extruding edges to get the general shape. Then, I go in and add detail with the split poly tool. Starting at the front, ending at the back.
Now with NURBS I would start with the bonnet, and go down, do the lights... etc. You need to be familiar with the fillet tool and the stitch tool to be able to make a good NURBS car.
Also with nurbs what you should do is create the cage of NURBS first. Think of how you want the NURB patches seperated... etc.
This is all of the very general stuff I could think of right now. I hope it helps
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