How push the object?

Have a look here:

http://docs.unity3d.com/Documentation/ScriptReference/Rigidbody.AddForce.html

Just add some force to it, or set velocity directly (if you do not want acceleration process to be here):

rigidbody.velocity = new Vector3(speedX, speedY, speedZ)

Please note that you can always assign Physics Material to rigidbodies and tune up dynamic and static friction to customize how long does it take a body to completely stop. This may add some realistic properties to your stuff.

You can add force to rigidboy with simple line of code like, rigidbody.AddForuce(Direction); or rigidbody.velocity = direction to the gameObject it will move in the direction. Go through the Unity3d scripting manual