I need a health script.

I am currently creating a fps, and I need a script that will help me monitor health. All the scripts I have used just don't work. I am an average programmer, and am somewhat new to C#. The last script I used, just coulden't reference a GUI text. (I'm in Unity 5.3.1) I don't know where to put the heath script, how to display GUI, or how to create AI. Please, I'm just looking for a little help. If you can give me a script in C# that I can use, it would help out a ton. JavaScript will also work, but C# is preferred. If you need to know anything aboutmy program for extra info, I will supply it. Here are a few scripts i have, and what they do.

The character controlling mechanic:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPlayerController : MonoBehaviour {
 
     public float movementSpeed = 5.0f;
     public float mouseSensitivity = 5.0f;
     public float jumpSpeed = 5.0f;
 
     float verticalRotation = 0;
     public float upDownRange = 60.0f;
 
     float verticalVelocity = 1.0f;
 
     CharacterController characterController;
 
     // Use this for initialization
     void Start () {
         Screen.lockCursor = true;
         characterController = GetComponent<CharacterController>();
     }
 
     // Update is called once per frame
     void Update () {
         // Rotation
 
         float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensitivity;
         transform.Rotate(0, rotLeftRight, 0);
 
 
         verticalRotation -= Input.GetAxis("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
         Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation, 0, 0);
 
 
         // Movement
 
         float forwardSpeed = Input.GetAxis("Vertical") * movementSpeed;
         float sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
 
         verticalVelocity += Physics.gravity.y * Time.deltaTime;
 
         if( characterController.isGrounded && Input.GetButton("Jump") ) {
             verticalVelocity = jumpSpeed;
         }
 
         Vector3 speed = new Vector3( sideSpeed, verticalVelocity, forwardSpeed );
 
         speed = transform.rotation * speed;
 
 
         characterController.Move( speed * Time.deltaTime );
 
         if (Input.GetKeyDown (KeyCode.Escape)) {
             Screen.lockCursor = false;
         }
     }
         
 }

Here is the shooting mechanic: using UnityEngine; using System.Collections;

 public class FP_Shooter : MonoBehaviour {
 
     public GameObject bullet_prefab;
     float bulletImpulse = 60f;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetButtonDown("Fire1")) {
             Camera cam = Camera.main;
             GameObject thebullet = (GameObject)Instantiate (bullet_prefab, cam.transform.position + cam.transform.forward, cam.transform.rotation);
             thebullet.GetComponent<Rigidbody>().AddForce (cam.transform.forward * bulletImpulse, ForceMode.Impulse);
 
         }
     
     }
 }
 

I have a prefab- "Bullet" - the object used as a bullet.

And this is the script for the GUI to display ammo. (NEVER WORKED, COULD NOT SEE GUI TEXT)

 var health = 300;
 var ammo = 100;
 
 function Update(){
 this.GetComponent.<GUIText>().text = "Ammo: " + ammo.ToString();
 }

Any help at all would be great. Thanks.