Subdomains in cPanel
From Lunarpages Web Hosting Wiki
A subdomain would be a domain name that has a prefix in front of it representing the folder that it came from. For example, If you setup yourdomain.com/blog/ as a subdomain the new address for it would be blog.yourdomain.com.
How to Setup a Subdomain in cPanel
The first thing you need to do is login to the cPanel for your hosting account. Then on the front page, scroll down to the Domains section and click the link for Subdomains.
To create your subdomain, first select the domain name from the drop-down menu, and to the left enter the subdomain name. Next click your mouse in the Document root box and it should automatically fill the path in.
A folder (directory) is automatically created with the same name as the new subdomain. It is located in the root (public_html) directory. This is where you would upload the files for the subdomain, in this case http://blog.yourdomain.com.
Redirect a Subdomain in cPanel
If you like, you may redirect a subdomain to any URL of your choice. To do this, login to cPanel and scroll down to the Domains section and click on Subdomains. Under Modify a Subdomain find the one you want to change and click on the Manage Redirection link to the right.
Enter the URL where you wish the subdomain to point. If it is pointing to a specific page, make sure you include a trailing "/". Once you have done that, review your information and if it is all correct then hit the Save button.
To remove the subdomain redirect, click on the Disable Redirection button.
Delete the Subdomain in cPanel
To delete a subdomain, login to cPanel click on the Subdomains link. Find the subdomain you want to delete and click the Remove link.
Please Note: cPanel does not support adding subdomains to addon domains at this time.