I am making a FPS and I was wondering how i could make it so when the enemy gets close enough he starts to deal damage.

In my game i want it so that when he gets in a certain distance close to the player he deals damage. I'm not sure if i should use OnTriggerEnter or OnCollisonEnter or how to exactly get this box collider on a different object. Im just not sure how to specify the players collider right now what ever it hits it makes the player take damage. Here's what i have so far:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class EnemyDamage : MonoBehaviour {
 
     public float StartHealth = 100f;
     public Text HealthNumber;
     public Image HealthBar;
 
 
     void Start () {
         
     }
     
     void Update () {
         HealthBar.fillAmount = StartHealth / 100;
         HealthNumber.text = StartHealth.ToString();
     }
 
     void OnTriggerEnter(Collider other)
     {
         StartCoroutine(Damage());
     }
 
     void OnTriggerExit(Collider other)
     {
         StartCoroutine(WaitForDamage());
         StopCoroutine(Damage());
     }
 
     IEnumerator Damage()
     {
         StartHealth -= 5;
         yield return new WaitForSeconds(3);
         Update();
     }
 
     IEnumerator WaitForDamage()
     {
         yield return new WaitForSeconds(3);
         Update();
     }
 }