C# How to apply damage to enemys? Shooter

Hey, I am kindly new to Unity3d and c# and I don´t understand how I could let a bullet deal damage to an other object. I´ve seen a lot of JS tutorials but no for C#. I´ve tried to use them, but it wont work. My code I´ve tried: For the Bullet (Prefab)

 // Update is called once per frame
     void Update () {
     
         transform.Translate (0, 0, speed);
 
 
 
     }
 
     void OnTriggerEnter(){
 
         if(gameObject.CompareTag("enemy"))
         {
             gameObject.SendMessage("OnDamage", damage);
 
         }
 
     }

And the one for the Enemy: public class enemyhealth : MonoBehaviour {

     public int health = 100;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void OnDamage(){
 
         health--;
 
         if (health <= 0) {
 
             Destroy(gameObject);
 
         }
     }
 
 }

Whats not clear to me aswell is, what type of Collider I should use (Box Collider, CapsuleCollider...) Or doesnt it makes a difference?

As I understood, Rigidbody is used for an other "technic" to make damage, am I right?