I want to restrict access to certain areas of my web site to authorised users only

There are two ways of doing this.
The first and simplest way is to use the "Protected directories" feature in the control panel (recommended for normal users).
The second and more advanced way is to upload pre-generated .htaccess and .htpasswd files to the directory which you want to protect. If you are unable to generate these from your end, please contact Support and we can help you generate these files.