[Stiglr]

On your first extrusion, the problem is that your surfaces are facing inward, and are two-sided.

To fix that:

1) Select entire object in Object Mode.
2) At the bottom left of the GUI, click the 1S button.
3) Switch to Surface mode. (all surfaces will be colored as "selected")
3) In the Surface menu, select Flip Normal.

As for the shape of the car itself, you're not capturing the shape properly. It is not possible to correctly model a car by tracing a polygon in a side view and extruding. It may match the drawing in the side view, but it certainly won't match the drawing in the front view. Doing it that way, you will miss all the contours, the least of which, the difference in width between the top of the car and the body.

You will need to learn something about basic 3D box modeling concepts before you get better results. Perhaps do a Google search for "box modeling techniques"