I guess the first thing to do is grab some paper, a bunch of pens and pencils and start drawing. Find some good sized clean photos for reference of your favorite car and start drawing. Post what you have so we can see where you are and what you can work on (if anything).
One of the best things you can buy is the book "How to Draw Cars Like a Pro" by Thom Taylor. Also, go to
www.cardesignnews.com and check out the tutorials in the "studio" section.
Have fun, don't be affraid to make a mess and good luck!