Thanks for your answers everyone!!
But I still don't get it

Please don't judge me, I'm normally not that stupid
I realize how annoying I must be, but this is the tutorial I've been looking for for ages. I' searching for the solution for so many days now, so any help is really really appreciated.
I have 2 questions:
This path must be full path to your folder. If you are on Windows box you should use Windows style path - C:\your\files , if you are oh Unix (Linux) box - /your/files
I don't test the PHP files offline, I immediately test them online, so does the C:\...\... work? Let's say that my files are hosted a Geocities (I know they don't support php, but that doesn't really matter). Should I then use the directory of the files there? Like for example, if I have a map 'files' and there the files are located in, then the directory is just 'files', or should I use the directory of my compueter, even if they don't have the same kind of .../.../... as the one on my computer?
And my second question: I tried to try out this tutorial just by copying the files described in the tutorial. And then I made an home.php, about.php,...
Should I also put something else in the index file or edit something (besides '/path/to/your/site' and 'about.php'?)
I copied this:
<?php
ini_set('include_path', '/path/to/your/site');
switch ($_GET['page']) {
case 'about':
include('about.php');
break;
case 'default':
include('home.php');
break;
}
?>