how to access a screen(separate obj in my app).

i think you would be better with a 3D text so you can place it in the position of the screen. For the buttons i would suggest using Physics.Raycast so you can get what object the user hits ad use that in your programming. More info on this can be found here :- http://docs.unity3d.com/Documentation/ScriptReference/Physics.Raycast.html for example

 var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 var hit : RaycastHit;
 if (Physics.Raycast (ray, hit, 100)) 
 {
     if(hit.gameObject.name == "button 1")
     {
         Debug.Log("Hit button 1");
     }
 }