How can i get a scoring system?

You didn't provide much information about your game or some code so here's a dirty example :

1. Test Scene setup Attach

2. ScoreManager.cs to a new GameObject

3. Attach EnemyScript.cs to a cube

4. Press Play Click 10 times with left mouse

Enemy gameObject should be Destroyed and Score should be set to 1

ScoreManager.cs

 using UnityEngine;
 
 public class ScoreManager : MonoBehaviour {
 
     public static int Score;
     public int _score { 
         get 
         { 
             return Score; 
         } 
         set
         {
             Score = value;
         }
     }
     
     void OnGUI(){
         GUILayout.Label ("Score : " + _score.ToString());
     }
 }

EnemyScript.cs

 using UnityEngine;
 
 public class EnemyScript : MonoBehaviour {
     
     static int Health = 10;
     
     void Update () {
         if (Input.GetMouseButtonDown (0)) {
             Health -= 1;
         }
 
         if(Health <= 0){
             ScoreManager.Score += 1;
             Destroy(gameObject);
         }
     }
 }