You can create any files and directories in your /www/ directory. If you want to provide a web site just under your domain name, e.g. http://testing.me.uk/ then there are some restrictions on directories you can use - you cannot use bin, etc., lib, tmp, or usr. These are also unavailable as sub domains (e.g. you cannot have http://bin.testing.me.uk/. See Webhosting Redirecting for redirection if you want http://testing.me.uk/ to redirect to http://www.testing.me.uk/
These restrictions are because the system creates a fake root environment for your scripts to use. This is set up automatically the first time you use ftp to access the site.
In any directory, including the main /www/ directory, you can have an index (often called index.html ). This is displayed if someone goes to the site or directory without specifying a file name. e.g. http://www.testing.me.uk/fred/ will access /www/fred/index.html on your web site.
You can have an index.html (normal web page index), or index.htm, or index.cgi, or index.gif. You can also use index.wml for WAP phone use.
Note that any subdomains for which you have DNS entries appear as directories in your web space. e.g. http://www2.testing.me.uk/ is the same as http://testing.me.uk/www2/ and is in the /www2/ directory. It is sensible to make links to other directories absolute (e.g. with http://www.testing.uk/, etc.) so that viewing in different ways does not cause problems.
The file password.txt in any directory is special - see passwords. See more here: Webhosting Password Protected Areas
The directory called weblog is special as it is used for web logs, see: Webhosting Weblogs
The file errlog.txt in the top level directory is special, see Webhosting Perl and CGI Scripts
Custom error pages
You can create your own customised error pages if you wish. Simply create a page that is the error number followed by .html, e.g. 404.html will be the "page not found" error page. This applies to any requests in the same or lower directories unless a lower directory also has a 404.html.
You can create a file called 'https' in your web space. That page, and any subdomains of it, should then automatically redirect to HTTPS. Note that the Let's Encrypt certificate may not yet be correct if the domain has very recently been added to the system, but this should fix itself within a few hours.