When you create a new Modern UI(Metro) Windows 8 Project in Visual Studio , a default start page MainPage.xaml will be created automatically along with other necessary files needed for running the Windows 8 App.

If you want to change the default start page of the Windows 8 App from MainPage.xaml to another file for example (firstpage.xaml) , you can do that by following the below steps.

The App.xaml.cs has the Onlaunched event where the initial start page is defined . By Default , the MainPage is included .

How to change the Start page of the Windows 8 App in Visual Studio 2012 ?

You can create a new blank xaml page and replace “MainPage” with the newly create page name . For example , if your new pagename is firstpage.xaml , then replace MainPage with the firstpage like the way shown in the below sourcecode sample.

// Invoked when the application is launched normally by the end user.  Other entry points
// will be used when the application is launched to open a specific file, to display
// search results, and so forth.
//Details about the launch request and process.
protected override void OnLaunched(LaunchActivatedEventArgs args)
{
            // Do not repeat app initialization when already running, just ensure that
            // the window is active
            if (args.PreviousExecutionState == ApplicationExecutionState.Running)
            {
                Window.Current.Activate();
                return;
            }

            if (args.PreviousExecutionState == ApplicationExecutionState.Terminated)
            {
                //TODO: Load state from previously suspended application
            }

            // Create a Frame to act navigation context and navigate to the first page
            var rootFrame = new Frame();
            if (!rootFrame.Navigate(typeof(firstpage)))
            {
                throw new Exception("Failed to create initial page");
            }

            // Place the frame in the current Window and ensure that it is active
            Window.Current.Content = rootFrame;
            Window.Current.Activate();
}

Now , run the application . You should see the Application running with the new start page in the Emulator now