How to go to a new gui.window by clicking a gui.button?

I'm still learning Unity3d and I have made a main menu out of gui window and its buttons made of gui buttons. When i click a button, i want to go to a new window. How is that? thank you for the answers! really appreciate it. This is my main menu scode:

using UnityEngine; using System.Collections;

public class cameraScript : MonoBehaviour {

 private Rect mainmenu = new Rect (0,0,Screen.width,Screen.height);
 
 public Texture start_btn;
 public Texture learn_btn;
 public Texture about_btn;
 public Texture ext_btn;
 public GUISkin mainSkin;
 
 void OnGUI()
 {
     GUI.skin=mainSkin;
     mainWindow();
 }
 
 void mainWindow()
 {
     mainmenu = GUI.Window(0, mainmenu, mainwFunc, "");
 }
 
 
 void mainwFunc(int winID)
 {
     if(GUI.Button(new Rect(50,150,150,50),start_btn))
     {
         AutoFade.LoadLevel(1, 0.25f, 0.25f,Color.black);
     }
     else if(GUI.Button(new Rect(50,200,150,50),learn_btn))
     {
         
     }
     else if(GUI.Button(new Rect(50,250,150,50),about_btn))
     {
         
     }
     else if(GUI.Button(new Rect(50,300,150,50),ext_btn))
     {
         
     }
 }

}