Just read up on poly modeling + cars. (Yes, there will be a ton of stuff to go through before finding one that seems approachable.) Carrara pretty much follows that concept and approach. (Unless you play around with lofting or whatever it is that is more NURBS-like.) Also the level of detail or realism you're going for will affect how you go about modeling. And even then some people differ on whether they mostly use a top-down approach (shape out the model first, and cut into parts) or bottom-up (build each piece individually and assemble into the car.)

If you find modeling in Carrara to be a little bit slow or bothersome, Carrara imports .obj just fine. I'd recommend Wings 3D for modeling and exporting to .obj, if you haven't tried it yet. (It's free btw.) Never hurts to keep a few options open if it isn't to be 100% all Carrara. (But if it's some kind of challenge that requires 100% Carrara, then good luck on your quest!)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.