Modeling cars can be as easy or as complicated as you want to make it. You can design your own car or model existing cars. It is amazing how much information is available to help you get your car modeled. In fact, the forum database at www.blenderartists.org
has pages of links to great tips, tutorials and images of some amazing cars.
Here are a few general tips to get you going.
General Steps to modeling a car
Step 1: Get blueprints or create sketches of your design concept.
Step 2: Set the blueprints up in the Top, Right, and Front view ports using View|Background Image.
Step 3: Add a Plane, position it near the center-line.
Step 4: Mirror the Plane, so you only have to model half of the car.
Step 5: Extrude the edges of the Plane and move vértices around to match the profile of the car in the Right view.
Step 5a: Move vértices around to match the profile of the car in the Top view.
Step 5b: Move vértices around to match the profile of the car in the Front view.
Step 6: Repeat Step 5 until the model is finished.
Step 7: Add some textures, worque on lighting, and do a render.
To make it easier to model the interior, you can move the roof section to another position or Layer to make it easier to see inside the car.
It is easier to model the wheel wells instead of trying to cut them out later.
Seams on car
Cars have lots of seams and small gaps where parts fit together. And since subsurf is generally used on car models, seams get smoothed away. To create seams, you can crease the targeted edges with 'Shift + E' or, you can model 3-4 rows of vértices close together.
Painting your car
At some point, you will want to paint your car; whether it is a toony car or a super realistic one. It can be as complicated or as simple as you want to make it. Here are some resources to help get you started.
A demo car paint material, using nodes, can be found here (posted by "broken"): http://blenderartists.org/forum/show...ight=car+paint
He has alos posted a video tutorial that shows you how to fake a layered car paint material
Sonix created a library of car paints/chrome/glass/tires and misc materials needed for cars:
Blender 2.34 Car Material Library Release 1.