GameObject.Find : why not working?

Hello everyone, maybe it will look like a stupid question to all of you, but until now I only programmed in darkbasic and I'm just starting with Unity, so please, make me understand why this code doesn't work. I want to have access to an object (child so it moves together) so I can use it's collision as a trigger. In the code it only rotates with Z because I was trying to see when the access worked, but it didn't, it continued saying ""NullReferenceException: Object reference not set to an instance of an object playerBehaviour.Update () (at Assets/Scripts/playerBehaviour.js:44)" I read and re-read the pages on the other object access and the GameObject.Find(), but I couldn't understand what I did wrong, neither figure it from the message. Please help, I'm used to a syntax much simpler, so it may look easy but it isn't for me. Code not working:

 #pragma strict
 
 public var giocatore : Rigidbody;
 private var velocita : float = 1;
 private var collinf : GameObject;
 
 function Start () {
 
 giocatore = GetComponent.<Rigidbody>();
 collinf = GameObject.Find ("Giocatore/CollisoreInferiore") ;
 
 }
 
 function Update () {
  if ( Input.GetKey (KeyCode.LeftShift) ) {
   velocita = 2;
  }
  else {
   velocita = 1;
  }
  if (Input.GetKeyDown (KeyCode.Space) ) {
   giocatore.velocity = Vector3 (0, 5, 0);
  }
  if ( Input.GetKey (KeyCode.W) ) {
   giocatore.transform.Translate (0, 0, velocita*Time.deltaTime);
  }
  if ( Input.GetKey (KeyCode.S) ) {
   giocatore.transform.Translate (0, 0, -velocita*Time.deltaTime);
  }
  if ( Input.GetKey (KeyCode.D) ) {
   giocatore.transform.Translate (Time.deltaTime, 0, 0);
  }
  if ( Input.GetKey (KeyCode.A) ) {
   giocatore.transform.Translate (-Time.deltaTime, 0, 0);
  }
  if ( Input.GetAxis ("Mouse X") > 0 ) {
  giocatore.transform.Rotate (0, 180*Time.deltaTime, 0);
  }
  if ( Input.GetAxis ("Mouse X") < 0 ) {
  giocatore.transform.Rotate (0, -180*Time.deltaTime, 0);
  }
  
  if ( Input.GetKey (KeyCode.Z) ) {
   collinf.transform.Rotate (0, 0, 180*Time.deltaTime);
  }
  
 }