A sub domain is an additional domain name that is based on your current domain name. 

An example of a sub domain of would be:

php.mydomain.com.

This feature is useful if you want to divide up your site into different sections.

How it works: 

You can create unlimited sub domains using your control panel.

Choose a name for your sub domain and enter it in the name filed as shown below:

Click the "add" button and your sub domain will be added automatically.

Note: It may be several minutes before the sub domain activates on the server.

You have the option to redirect newly created sub domains to a different web site.

To upload files to your new sub domain, look for the new folder that has the same name as your sub domain in your public_html folder.

For example: http://php.mydomain.com is the same as http://mydomain.com/php

Therefore, you should upload to your subdomain accordingly.

1)your public_html folder. 

2) Select your sub domain folder and upload your files using the built in file manager. You may also upload files using your FTP client or Microsoft FrontPage.