I would suggest taking a the time to watch a few tutorials, you will be able to help yourself alot more just by watching some basic ones. UnityStudent check out this link it will cover exactly what you want and probably other things you might find your self thinking of implementing.
Scripting Reference : OnCollisionEnter
You can read up on Destroy(); here: Unity - Scripting API: Object.Destroy
It has a feature already where you can set the delay of the destruction, to check collisions you should use the OnCollisionEnter function.
OnCollisionEnter (des : Collision)
if(des.gameobject.tag == “destroyer”)