Well, I am pretty new at modding my self however I have tried my hand at car modding and was pretty successful. For modeling I use "Zmodeler2" and of course textures comes from Photoshop CS6. I have yet to figure out how to 3D model my own cars so I just use "unlocked" models provided by BxBugs123. What "unlock" means that some models are unrestricted models given by their authors for public use. The downfall of using Zmodeler is that the free version will not allow you to import your own models so buying it is almost a must keyword almost.
Here are some links that may help:
Development models: http://gtapolicemods.com/downloads.php?do=cat&id=136
After I get a hang of modding, I am planning to do a tutorial video since there is such a lack of these.
Hope I helped, good luck!