if you mean modeling in 3D,
then first of al, you need a 3D prog, like Maya, cinema, studio max, blender, bryce and so on(the last three are free).
then you should first do some very easy 3d-Modeling tuts(to get the hang of it) over at pixel2life, or such an site. there are some great tut's here two under the recource tab.
then you should start with searching some blueprints of you're car, or make them yourself....(if you want to make youre OWN car)
then you should load those blueprints in as imageplanes, and you're ready to model you're very own car.
do mind that the getting the hang of it takes a while, you won't be able to get you're car done by tommorow.
no really not