Adding components to 3d models

I want to add a 3d model of a car to my game and controll it. Now i am using a cube from unity as player, with a rigidbody and his collider, so it stays on the ground (plane). But when i import my 3d model, y can add collider to it, but when i add rigidbody component the car go through the ground without colliding. I put a mesh filter and a mesh colllider to the car, but when the rigidbody is turned it doesnt work.