Collision between two sphere game object

In my game scenario , i have two sphere game object. In that i have one sphere game object is on statis position and other sphere game object is used as bullet. For that i have created prefab of sphere game object. And i have put empty game object in my 2D scene and instantiate my bullet prefabe on that empty game object position.

Now when i shoot bullet i want to detect collision. I have tried so many methods but none of them works for me.

My code:

 function Update ()
 {
  
     var spawnPoint : GameObject = GameObject.Find("Bullet");
   
     if(Input.GetMouseButtonDown(0))     
     {     
          if(spawnPoint != null)
          {
               var hit  : RaycastHit ;
               var ray : Ray  = Camera .main .ScreenPointToRay (Input .mousePosition );
               if(Physics.Raycast (ray, hit))
               {     
                        
                        shootProjectile = Instantiate(bulletSphere, spawnPoint.transform.position,spawnPoint.transform.rotation);
                      shootProjectile.rigidbody.useGravity = false;
                      var tempx = hit.transform.position.x;
                      var tempy  = hit.transform.position.y;
                    
                      shootProjectile.rigidbody.AddForce (shootProjectile.transform.TransformDirection(new Vector3(tempx,tempy,0) * 8000 * Time.deltaTime));        
                      //if(hit.collider.gameObject.tag == "Enemy")
 //                     {
 //                         Destroy(hit.collider.gameObject);
 //                     }                 
                      
               }    
          }
      }
 }

Method i have tried :

OnControllerColliderHit (hit : ControllerColliderHit)

OnCollisionEnter(other : Collision )

Which collision matrix do i need to use?

please please help me to solve my this problem. Thanks in advance. :-(