How to make a "Click to Continue" text message?

This ought to do it for you. This will cycle through a list of messages you create in the Editor. I've tested the code and it works. You can add things to make it prettier, format the text, or position the box, but I'll leave that up to you.

 using UnityEngine;
 using System.Collections;
 
 public class TextOnDemand : MonoBehaviour {
 
     public string[] Messages; // Fill them in in the Editor, or you can figure how to load them
     [HideInInspector]
     public int CurrentMessageIndex = 0;  // Default to first
     public bool ShowMessages = true;
 
     // Use this for initialization
     void Start () {
         // Load messages, if you did not make them in the Editor
     }
     
     // Update is called once per frame
     void Update () {
         // Handle Moving to next message on mouse click
         if (Input.GetMouseButtonUp(0))  // Left Mouse Click to move forward
             CurrentMessageIndex++;
         if (Input.GetMouseButtonUp(1))  // Right Mouse Click to move backward
             CurrentMessageIndex++;
 
         // Turn off messages if we get t othe last one.
         if (CurrentMessageIndex > Messages.GetUpperBound(0))
             ShowMessages = false;
 
         // Bounds checking
         CurrentMessageIndex = Mathf.Clamp(CurrentMessageIndex, Messages.GetLowerBound(0), Messages.GetUpperBound(0));    
     }
 
     void OnGUI()
     {
         // Now show the text
         if (ShowMessages)
             GUI.TextArea(new Rect(10, 10, 500, 200), Messages[CurrentMessageIndex]);
     }
 }