How to destroy a gameobject when I collide with it?

Something like this attached to the object you want to destroy?

 function OnCollisionEnter(col : Collision)
     {
         if(col.collider.gameObject.name == "LawnMower")
         {
             Destroy(gameObject);
         }
         else if (col.collider.gameObject.name != "LawnMower")
             {
                     //Something else
             } 
     }

End level, something like this attached to the Player:

 var grassObjects : GameObject[];
 
 function Update()
 {
      grassObjects = FindGameObjectsWithTag ("Grass");
      var count = grassObjects.length;
      if ( grassObjects == 0 )
      {
           //End level by showing end screen or loading next etc
           //Debug.Log("No Grass Left");
           //Application.LoadLevel ("NextLevel");
      }
 }

Make sure you tag your grass as "Grass" in the inspector. Add a tag by increasing the Elements by one and adding it in the new empty field.

You probably will want to do this periodically rather than every frame to avoid chewing up your cpu cycles. Find functions are very expensive. Try looking in to this:

Unity invoke repeating

Its all untested, I could be wrong. Let me know if it throws up errors :)