To make a subdomain, log in to your control panel and click "domain management" inside of domain management you will see a place to manage and add subdomains. If you do not have a control panel, we can set the subdomain for you, submit a ticket to do so.
Important note:
To create a subdomain, cPanel asks for the name you wish to use. MAKE SURE the subdomain name you give it doesn't have a folder already named like it. If it does, the subdomain will not work. Example: If you want to create the subdomain testsub.ozcart.com, the subdomain name is going to be testsub. If you already have a folder in your FTP named testsub you need to delete it or rename it to something different before you create the subdomain with the test name from cPanel.