Do a Google search for car rendering. Sketch as much as you can, especially existing designs.
99% of getting this portfolio together is your own internal drive. With or without a mentor, you will learn by making 10,000 mistakes and horrible drawings. With your background, you will recognize them and be able to avoid them again.
"Our inventions are wont to be pretty toys, which distract our attention from serious things. They are but improved means to an unimproved end, an end which it was already but too easy to arrive at...." Thoreau