What (or who) is directing you to set up an alias? Somebody else that's trying to explain how to configure mail addresses, per your other recent question? (I've posted a response over there, too.)
New account entries are a reference to the SMTP mail server and to the associated the POP or IMAP server, the credentials and the ports involved for the SMTP and POP/IMAP servers.
If you want to send messages from multiple addresses (and selecting a "from" address in the message compose window), you can do that by creating a comma-separated list of email address in the email address field. Mail > Preferences > Accounts (for each account you have) > Email address > {insert a comma-separated list of email addresses you want to send from, when you compose messages}
Apple has available a tour of the OS X interface, if you're interested. There are also various third-party books available, if that's how you prefer to learn. Apple also has some training available at their various Apple stores, though that seems to vary by location.