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))
	{
		
	}
}

}

By Application.LoadLevel(1) you can go new scene,
But if you are trying in same scene then firstly you have to make your GUI WINDOW then by coding or some boolean values you can do it.