Step-1: Login into your cPanel
Step-2: Go to the domains section and click on Subdomains
Step-3: Type in the name of the subdomain and select the domain to create it under. It will automatically create a folder for the subdomain in your public_html folder.
Step4: Click the Create button. You will then see a message stating it "has been created."
Congratulations, you have added your first subdomain!