I still don’t understand the difference between a domain and a sub-domain.

A subdomain is like forums.cpwebhosting.net where the forums part is the subdomain.

A domain is cpwebhosting.net where it has nothing before it.

Domains = contain the DNS, subdomains can’t use their own DNS unless you have a script for it.
Subdomains are usually where you put added on features to a normal domain, like a forum or gallery.

subdomain is a name you can use before your domain, generally where you see www on most sites.

To create subdomains you need a domain to begin. Also how to make them would depend on the control panel your webhosting provider provides along with the option given by your web hosting provider to create them, check with your webhoster to check if they offer it as an option and which control panel they use to receive a proper response

if you have a forum at www,yoursite.com/forum, you can create a subdomain called forum and link to it. then it would be forum.yoursite.com.