Help with a health bar

Hi there. I am working on a 2.5D platformer in Unity 5.5.1. I have a script for an auto-healing health bar, but I want it to work so that the player character will take damage when attacked by an enemy. So far, I only have it programmed so that damage can be taken with the press of a button. Here is my script: (it's in Java)

var ls:UnityEngine.SceneManagement.SceneManager;

var health = 300; var damaged = false;

function Update (){ if(Input.GetButtonDown("C")){

 playerDamage();
 }
 if(damaged == false){
     if(health < 300){
     health += 1;
     }
     if(health <= 1){
     ls.LoadScene("MissionFailed"); 
     }
 }

}

function playerDamage(){ damaged = true; health -= 30; yield WaitForSeconds(1); damaged = false; }

function OnGUI(){ if (damaged == true) { GUI.backgroundColor = Color.red; } else { GUI.backgroundColor = Color.green; } GUI.Button(Rect(20,20,health,20), "health"); }

Any help is always appreciated, so thank you :)