for converting models / creating GTA vehicles you gonna need a registered version of zmodeler2 (it's like 60$ or smth), some guides for it are on the zmodeler homepage. i dont know of a tutorial about the whole converting process tho, i dont think there is one.
i'd say creating a car isnt easy at all, takes lots of work and knowledge and most of all, time.
Nope there aren't any tutorials about the whole converting process. There are some little tutorials but none about the whole converting process.
Smokey is right. Converting cars isn't easy at all. you need spend lots of time, patience and reflection!
Oh, one important thing is to discover things yourself, like, if you don't know how to do something, try different thing and keep trying. Ask for help only if you had already all tried and you can't fix the problem. That helps you a lot in the understanding of the program.
You will need a program like Photoshop or Gimp for the textures too.
Hope i helped!