Network Not Updating On All Clients PLEASE HELP!!!

I'm very new to networking in Unity so sorry if this is an easy question. When I try to change text and delete gameobjects it only shows up on the host. I know everything on this script might not be the most efficient way but I just want to know, not how to make it shorter. And I also know I might need to change to a NetworkBehaviour for SyncVar or ClientRpc. By the way, shouldStart is turned on in other scripts when a game starts. Heres the code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using UnityEngine.Networking;
 
 public class GameController : MonoBehaviour {
 
     public Text timer;
     public Camera theCamera;
     public GameObject removalWall;
 
     [HideInInspector]
     public bool shouldStart = false;
 
     void Start ()
     {
         timer = GameObject.FindGameObjectWithTag("Timer").GetComponent<Text>();
         theCamera = GameObject.FindGameObjectWithTag("MainCamera").GetComponent<Camera>();
         removalWall = GameObject.FindGameObjectWithTag("RemoveWall");
     }
 
     void Update ()
     {
         timer = GameObject.FindGameObjectWithTag("Timer").GetComponent<Text>();
         theCamera = GameObject.FindGameObjectWithTag("MainCamera").GetComponent<Camera>();
         removalWall = GameObject.FindGameObjectWithTag("RemoveWall");
 
         if (shouldStart == true)
         {
             StartCoroutine(StartCountdown());
             shouldStart = false;
         }
     }
 
     IEnumerator StartCountdown ()
     {
         timer.text = "10";
         yield return new WaitForSeconds(1);
         timer.text = "9";
         yield return new WaitForSeconds(1);
         timer.text = "8";
         yield return new WaitForSeconds(1);
         timer.text = "7";
         yield return new WaitForSeconds(1);
         timer.text = "6";
         yield return new WaitForSeconds(1);
         timer.text = "5";
         yield return new WaitForSeconds(1);
         timer.text = "4";
         yield return new WaitForSeconds(1);
         timer.text = "3";
         yield return new WaitForSeconds(1);
         timer.text = "2";
         yield return new WaitForSeconds(1);
         timer.text = "1";
         yield return new WaitForSeconds(1);
         timer.text = "Go!";
         //removalWall.SetActive(false);
         Network.Destroy(removalWall);
         yield return new WaitForSeconds(1);
     }
 }