To add a subdomain (ex. mysubdomain.mydomain.tld) you will first need to login to your myCP. Once logged in, scroll down the to bottom to the Domain Manager and click the link for Add Domain.
On the next screen enter the subdomain you wish to add, and choose if you want it to have its own webspace or point to another website, and if you want the subdomain to have its own email (ex. email@example.com) or just use the primary domains email setup (firstname.lastname@example.org). And click Add.
You may get a warning about making sure that you are the owner of the domain, if you are, then no problem.
This will then setup a webspace for that subdomain that you can view in the Secure File Manager from your main myCP page. You can upload files via FTP using your normal FTP credentials of your account.