Hello Mr Socket.
AutoCad itself has a quite a good 3D package these days and given that is a program that you already are aware of, I'd recommend exploring that feature of it first.
The help files in it give fairly good synopsis of how to build objects, shade them and print them. Else there are a lot of free tutorials on the web.
If it is cool renders that you want you can then export these drawings to any one of a number of 3D drawing Rendering packages to develop them into a decent picture, for example 3DStudioMax (3DMax), Rhino, and Maya.
I typically draw elements of the car in AutoCad then import them into 3DMax
3DSMax is a program with all the bells and whistles, although this makes it harder to master in the long run than some of the simpler rendering packages out there it really produces good results and has a huge following and therefore support on the web. You can complete generate the 3d elements in the program, colour them, add lighting, even animate them into movies.
A quick search for some car specific tutorials for 3DMax brought up the following examples:http://www.carbodydesign.com/tutorials/software-tutorials.php?app=3DStudio%20MAXhttp://www.danielbuck.net/index.php?page=tutorialshttp://www.3d-palace.com/tmas/video.php?vid=58
It does take time, this is no quick fix to get super results, but you should be able to get something serviceable in reasonable time.
As Dr Goggles mentioned, one of the best aspects of the tool is the ability to move a viewpoint around the object that you are looking at so you can really get a feel for its shape. This is so handy when blocking out a car where you can quickly alter wheelbase, colour, shape, etc.
I must mention that I have an industry bias to these programs as I am a puppet animator and architect by trade and so these are the design tools I use regularly.
Other programs from different fields that you might be looking at would include something like SolidWorks which is from the engineering direction and can help set up all the CadCam tooling for you for the parts you wish to manufacture.
What do you want the program for? Pretty pictures or to work stuff out, or to actually build it with?
Hope this has been of help.