How to create a system of damage to a racing game

Well I'm starting a racing game and I already have the base game, but also wanted to know how to create a system of damage to the car and the object receiving the collision, the car would only dent in the party and I also bumped let loose some parts of the model.

I would also create a login system for who is not registered, could only play the demo. And finally how to create a selection screen for race cars

Since you giving us only some keywords and asked 3 question at once, I will give you only some keywords back.

And the final hint take a look at the FAQ