How can I make my gameObject find the nearest object with a specific tag and rotate to look at it?

Something along these lines should be what you're after:
(Edit : now tested & working!)

// the tag to search for (set this value in the inspector) var searchTag = "Respawn";

 var nearestDistanceSqr = Mathf.Infinity;
 var taggedGameObjects = GameObject.FindGameObjectsWithTag(searchTag); 
 var nearestObj : Transform = null;

 // loop through each tagged object, remembering nearest one found
 for (var obj : GameObject in taggedGameObjects) {

     var objectPos = obj.transform.position;
     var distanceSqr = (objectPos - transform.position).sqrMagnitude;

     if (distanceSqr < nearestDistanceSqr) {
         nearestObj = obj.transform;
         nearestDistanceSqr = distanceSqr;
     }
 }

 return nearestObj;

I was guided to the answer I was looking for, thank you very much! Here is the working code:

function Update () { var waypoints: GameObject[] = GameObject.FindGameObjectsWithTag("waypoint"); var closest: GameObject; var closestDist = Mathf.Infinity;

 for (waypoint in waypoints) { 
     var dist = (transform.position - waypoint.transform.position).sqrMagnitude; 

     if (dist < closestDist) { 
         closestDist = dist; 
         closest = waypoint; 
         } 
 } 
 transform.LookAt(closest.transform); 

}

hey man I noticed that Closest distance never resets and if it does it messes up my targeting here is what I have man see what you think it doesnt use LOOK AT because my code you can slow down the turret so if you wanted a huge battleship cannon you can do that with my code.

var closestDist = Mathf.Infinity; 

var attackRange = 30.0; var shootAngleDistance = 10.0;

var closest: GameObject;

function Update () {
var waypoints: GameObject[] = GameObject.FindGameObjectsWithTag("waypoint");

for(waypoint in waypoints) { var dist = (transform.position - waypoint.transform.position ).sqrMagnitude;

 if(dist <= closestDist)
 {
         closestDist = dist;
         closest = waypoint;       
 }else
 {
     closestDist += 100;
 }
 //Debuging ClosestDist
 if(Input.GetKeyDown("t"))
 {
 print("ClosestDist: " + closestDist);
 }


     var targetPoint = closest.transform.position;
     var targetRotation = Quaternion.LookRotation ( targetPoint - transform.position , Vector3.up);
     transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * 2.0);

     // If we are almost rotated towards target - fire one clip of ammo
     var forward = transform.TransformDirection(Vector3.forward);
     var targetDir = closest.transform.position - transform.position;

     if (Vector3.Angle(forward, targetDir) < shootAngleDistance && closest.gameObject.tag == "waypoint")
         SendMessage("Fire");


 }

}

just add one target transform and ignoring the Y axis and so this script can attached any object and can specify the transform

function GetNearestTaggedObject(searchTag : String,otherTrans: Transform) : Transform { // and finally the actual process for finding the nearest object:

     var nearestDistanceSqr : float = Mathf.Infinity;
     var taggedGameObjects : GameObject[] = GameObject.FindGameObjectsWithTag(searchTag); 
     var nearestObj : Transform = null;
 
     // loop through each tagged object, remembering nearest one found
     for (var obj : GameObject in taggedGameObjects) {
 
         var objectPos = obj.transform.position;
         var distanceSqr = (objectPos - Vector3(otherTrans.position.x,objectPos.y,otherTrans.position.z)).sqrMagnitude;
 
         if (distanceSqr < nearestDistanceSqr) {
             nearestObj = obj.transform;
             nearestDistanceSqr = distanceSqr;
         }
     }
 
     return nearestObj;
 }

Here's a method that I created to do this in C#. It also uses a helper method from my utility class.

     void SelectClosest()
     {
         const int maxSelectionDistance = 2;
 
         Selected = null;
         var closestDistance = Single.MaxValue;
         foreach (var selectable in GameObject.FindGameObjectsWithTag("Selectable"))
         {
             var distance = gameObject.DistanceSquaredTo(selectable);
             if (distance < closestDistance && distance < maxSelectionDistance * maxSelectionDistance)
             {
                 closestDistance = distance;
                 Selected = selectable;
             }
         }
     }
     
     public static float DistanceSquaredTo(this GameObject source, GameObject target)
     {
         return Vector3.SqrMagnitude(source.transform.position - target.transform.position);
     }