You can only have one domain per package, but of course you can have sub domains aplenty.
so each sub-domain would be http: // subdomain. yourmaindomain.co.uk
I've added spaces in the address to prevent the site turning it into a 'click here' link
Each subdomain must have its own document root inside the public_html directory, and browsers will load the index page that corresponds to the subdomain. So, all files relating to a subdomain called pcadvisor must go into a directory called pcadvisor inside the public-html directory.
You should never use your virtual server space to host websites for other people by the way - that's a practice that's fraught with danger.
Does that help?