Subdomains are secondary sections of your site, pretty much indipendent from the main domain; you can reach them directly, upload things directly to them using FTP and work on them as if they were totally separate entities. Here is a tutorial on how to create a subdomain in cPanel.
First, you have to login into your cPanel - url is generally located either at your-domain.com/cpanel or https:://your-domain.com:2083 - with the username and password your host gave you; the login page should show something like this.
Insert the required data, and you should see the main page of cPanel - similar to the one below.
Search until you find the section where it says Subdomains - like shown below - and click the icon.
When you are on the page, insert your subdomain's desired name in the first field - like in the example below - and click Create.
Creation will take a few moments, then you'll be take to a page confirming the creation of the subdomain; just click on the "Back" button; the result should be similar to below.
The subdomain is created; you should see its folder in your cPanel's File Manager.
Just enter the folder....an you are working on the subdomain; have fun! ^.^