Homing missle with raycast

hi i have been learning scripting and decide to try to make a lock on missile so Ive been looking and all the tutorials have a script that just look for the closest obj with the tag enemy and etc but what i want is a way to make the missile lock on to a target using a raycast from the middle of the screen hears the code ive been learning from `

 #pragma strict
  
 var _velocity : float = 10;
 var _torque: float = 5;
 var _target : Transform;
 var _rigidbody : Rigidbody;
  
 function Start()
 {
   _rigidbody = transform.rigidbody;
   Fire();
 }
  
 function Fire()
 {
    var distance = Mathf.Infinity;
    for (var go : GameObject in GameObject.FindGameObjectsWithTag("Enemy"))  {
       var diff = (go.transform.position - transform.position).sqrMagnitude;  
  
       if(diff < distance)
       {
          distance = diff;
          _target =  go.transform;
  
       }
    }
 }
  
 function FixedUpdate()
 {
  
   if(_target == null || _rigidbody == null)
      return;
     _rigidbody.velocity = transform.forward * _velocity;
  
     var targetRotation = Quaternion.LookRotation(_target.position - transform.position);
     _rigidbody.MoveRotation(Quaternion.RotateTowards(transform.rotation, targetRotation, _torque));
 }
  
 function OnCollisionEnter(collision : Collision)
 {
    Destroy(gameObject);
 }
  

p.s. i know how to use the raycast function but Im just confused on implementing it into the code.