Character Collision Trigger Text Message Help!

Hey guys, I am new to scripting and do not know how to go about creating this. That said, I want a text message to show up when my character walk's thru a collider and as he exit's out, the text should disappear. And apart from this, how can I make the text disappear after a certain amount of time?

Well, this is what I've done so far. Now the text appears after a certain interval and does not disappear. Would be really nice If someone could help me out. Thank you very much in advance.

 using UnityEngine;
 using System.Collections;
  
 public class mainText : MonoBehaviour 
 {
     public float letterPause = 0.05f;
     public AudioClip sound;
     public GUIStyle font;
     string message;
     string text;
  
     
     void Start () 
     {
        message = "Hello, World!";
        StartCoroutine(TypeText());
     }
  
     IEnumerator TypeText () 
     {
          float timeToWait = 2; 
 
          float incrementToRemove  = 0.5f;
 
               while(timeToWait > 0)
             
 
      {
 
           yield return new WaitForSeconds(incrementToRemove ); 
 
           timeToWait -= incrementToRemove;
 
      }
 
        foreach (char letter in message.ToCharArray()) 
        {
          text += letter;
          if (sound)
           audio.PlayOneShot (sound);
           yield return 0;
          yield return new WaitForSeconds (letterPause);
        }      
     }
  
     void OnGUI()
     {
        GUI.Label(new Rect(0, 0, 256, 1024), text, font); 
     }
  
     void Update()
     {
        if(Input.GetKeyDown (KeyCode.Return))
        {
          StopAllCoroutines();
          text = message;
        }
     }
 }