How to make car damage like in gta 4

I would start here: Unity official live training - Coding for the absolute beginner

If that's not the answer you were looking for: Get formal programming education, maybe Game Design studies, some internships, a few years of working experience, a kickass portfolio and then become a developer at rockstar to make a damage system like in GTA.

Make an Animation for destruction of the car then write some scripts to make it play this animation when the car hits something.You can find how to make an animation and to connect it with scripts on the internet.

This is not really a Unity task but a task of 3D Modelling.

You could implement deformations as BlendShapes and have doors etc as separate objects so they can be removed or destroyed.