How can to convert it on c#?

Here is the full conversion to C#. Some quick conversion rules on your code:

• all code from JS have to be encapsulated into a class; the file name must be accordingly with class name (ex: "Score.cs" for "class Score");

• Variable declaration become "type name = value" instead of "var name : type = value";

• "function" become "void" (not returning a type);

• "Rect" become "new Rect";

Other t$$anonymous$$ngs are the same.

If you have trouble with other.tag == "Coin" the reason is not related with conversion. Be sure the tag (case sensitive) "Coin" exists and your "other" object have the tag "Coin" assigned.

Note Destroy(other.gameObject) is distroying object $$anonymous$$tting the trigger. If you have a cube with "Is trigger" activated and Score.cs attached, and a FPS controller with "Coin" tag, then, when FPS $$anonymous$$t the cube, score will updated but the FPS will be destroyed, not the cube.

Score.cs:

 using UnityEngine;
 using System.Collections;
 
 public class Score : MonoBehaviour {
     
     
     int score = 0;
     string scoreText = "Score: 0";
     GUISkin mySkin;
     
     // Use t$$anonymous$$s for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void OnTriggerEnter( Collider other) {
         Debug.Log("OnTriggerEnter() was called");
         if (other.tag == "Coin") {
             
             Debug.Log("Other object is a coin");
             score += 10;
             scoreText = "Score: " + score;
             Debug.Log("Score is now " + score);
             Destroy(other.gameObject);
          
         }
     }
      
     void OnGUI () {
            GUI.skin = mySkin;
         GUI.Label (new Rect (10, 10, 500, 200), scoreText.ToString());
     }
 
 }