how to make bullets apply damage

what bullets are you using is it raycast or actual bullets if its a raycast one add this code

  var damage:float = 4;
     function Update () {
     //your fire bullet code(rate of fire,bullet--;//.etc)
     var hit:RaycastHit;
     //physics raycast code
 
 if(hit.transform.tag =="Enemy"){
 hit.transform.SendMessage("Damage",damage,SendMessageOption.DontRequireReceiver);
 
 }
 
 }

if it is an actual bullet then creaate sendmessage option in same way(using OtriggerEnter or similar ones)

To apply damage add this code to your enemy

 var life:float = 100;
 
 function Damage(damage:float){
 life-=damage;
 
 }