Explained

Now Lets Explain. Set the Default page to any page you want. So lets start, Case "Tutorials", means if ?page=Tutorials than show tutorials.php. We can change that to anything. So we can make it,

break; case "thetutorialsection": include('tutorials.php');

That means if the section is ?page=thetutorialpage, than it will show Tutorials.php.
You can add as many pages your site has. If you wish to change the ?page=, simply go to the top of the code and change switch($page).