Keep ur ISO low, especially when shooting still cars. Use adequate lighting and keep the sun behind you as much as possible. When in doubt, use a tripod and make sure you focus right where you need to. When shooting with my XT, I feel that I have to shoot a little under the middle of the exposure bar, otherwise they look too washed out and bright.
And shoot, shoot, shoot! Getting used to your camera will take time, do not let your DSLR discourage you in any way.
I'm by no means a great photographer, but I just keep it as a hobby so I'm not always bored. My first few photoshoots were not good at all - its a whole different ballgame. Dont RELY on post editing software, but use it to compliment your photos. You still need a good shot to make a great picture. This is with no flash lighting, and very very few touchups with Photoshop:
And I highly recommend you join
=http://www.photography-on-the.net
They have many pointers on there, and the professionals provide great advise.
Best of luck with your new camera!