How do you display a message to a player when he clicks on an object?

Update: I created a "LookAt" script:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class LookAt : MonoBehaviour {

 public bool canTake = false;
 public string lookMsg = "You see nothing special.";
 public string takeMsg = "You can't take that.";
 public string openMsg = "You can't do that.";
 public GameObject messageObject;
 string message;


 GameObject MessageManager;

 private bool showLabel = true;
 public void Start()
 {

// InvokeRepeating("ToggleLabel", 1,1); InvokeRepeating("ClearMessage", 1, 3); }

 //public void ToggleLabel()
 //{
 //    showLabel = !showLabel;

 //}
 public void ClearMessage()
 {
     message = "";
 }
 private void Update()
 {
     OnMouseDown();

 }

 void OnMouseDown()
 {

     if (Input.GetMouseButtonDown(0))
     {
         message = lookMsg;

         Debug.Log("Pressed left click.");
     }
     if (Input.GetMouseButtonDown(1))
         message = takeMsg;
         Debug.Log("Pressed right click.");

     if (Input.GetMouseButtonDown(2))
         message = openMsg;
         Debug.Log("Pressed middle click.");
 }

 void OnGUI()
 {
     if (showLabel)
     {
         GUI.Label(new Rect(250, 700, 1000, 1000), "<color=white><size=32>" + message + "</size></color>");

     }

 }

}

And a "MessageManager" script:

using UnityEngine; using System.Collections; using UnityEngine.UI; // include UI namespace so can reference UI elements using UnityEngine.SceneManagement; // include so we can manipulate SceneManager

public class MessageManager : MonoBehaviour {

 // static reference to message manager so can be called from other scripts directly (not just through gameobject component)
 public static MessageManager mm;


 // game performance
 public string pMessage = "Player Message:";


 // UI elements to control
  public Text UIPlayerMessages;


 // set things up here
 void Awake()
 {
     // setup reference to message manager
     if (mm == null)
         mm = this.GetComponent<MessageManager>();

     // setup all the variables, the UI, and provide errors if things not setup properly.
     setupDefaults();
 }


 // setup all the variables, the UI, and provide errors if things not setup properly.
 void setupDefaults()
 {
     // get the UI ready for the game
     refreshGUI();
 }

 void refreshGUI()
 {

     UIPlayerMessages.text = pMessage;
 }

 //send messages to the player
 public void DoMessage(string message)
 {
     pMessage = message;
     Debug.Log("Got to DoMessage.");

 }

}

I attached both to an object. Now, when I left click I get the look message, right click gives the take message and center click gives the open message.

My current issue is that I get the messages anywhere I click in the scene not just on the object.