need help with GUI healthbar

I have a want a healthbar in my game but i dont now how to make it can you plz help me. the health script is:

var health = 100.0; var dieDuration = 0;

function ApplyDamage( amount : float ) {

health -= amount;

if (health < 1) {
    Die(); 
} else {
    // <- (play "hurt" animation & sound here)
}

}

function Die() {

// <- (play "die" animation & sound)

yield WaitForSeconds( dieDuration );

  Application.LoadLevel(Application.loadedLevel);

}

and the damage script is:

var damage = 10;

function OnTriggerEnter (hit : Collider) {

if(hit.gameObject){

   hit.gameObject.BroadcastMessage("ApplyDamage", 
                                        damage, 
          SendMessageOptions.DontRequireReceiver);

}

}

I want a green healthbar that change to yellow when my character have 50 health and that change to red when the character have 15 health left.

Thx // Dummy7307

What a relevant name. There is already around 50 questions eg. this on this site with an answer that explains exactly what you want to do not to mention a butt load of tutorials that pop up if you use this magical search engine called google. Start off with some GUI literature to get you started you could also look at this GUI Guide but most of all you can research a question before asking it. The fact that there is this many and you have failed to find any shows that you really don't have two stones about you to look for an answer.