To Enable Directory Listing, edit your htaccess file (click here for a guide on doing that) and add this code to the TOP of the file:

Options +Indexes

Now when you visit your website, or a specific directory (eg: yourdomain.net.au/files) that doesn't have a index.php or index.html file, you will see a directory listing like this:

You can enable Directory Listing for specific directories by creating a file called .htaccess in them and adding the same code.