public static void Destroy(Object obj, float t = 0.0F);
The Unity docs have a good description of the Destroy method here. But in case you just want some quick code here it is.
To destroy a gameobject from within itself, just call
To destroy a gameobject from within after 5 seconds
To destroy a script instance from a gameobject
To destroy a component on a gameobject, in this example a rigidbody