How to add rigidbody to a imported gameobject of a modeling software?

I have imported a gameobject in my game which I created in a 3d modeling software. Now I would like control the movement of that gameobject so that as soon as it hits another gameobject it should stop his movement.

I have read that for this a gameobject needs a rigidbody. So how and where in the script can I add a rigidbody to that gameobject?

In the inspector add a rigidbody component.

You could learn a lot from these:

http://unity3d.com/learn/tutorials