How can I create a subdomain?

In the control panel, in the section Domains > Subdomains:

Clicking on "Create New", a pop up will appear asking you to name the new subdomain. The name selected will be followed by one of the domains in your account. Select the domain that you will use from the drop-down menu and proceed to generate a new subdomain. Example: blog.mywebsite.com.

Clicking on My Options, you will be able to determine to which address it should respond.