blinkingyellow is 100% right.
Addition to what he said, on easy thing to add to your drawing:
Dark areas, make them really really dark, this is quite easy to do with a 8b pencil or even a black pen. When you have dark areas on the drawings, it will just give it more of a realistic feel to it. When i say dark areas, i mean areas around the wheels, shadow under the car, parts of tires etc.
For materials, i usually just use something like one 2b pencil, one 4b pencil and one 8b or 9b pencil. And a good eraser!
I will try to add more tips later, perhaps with some picture to try to illustrate what i mean as my english has never been my strength!
