Please see How do I set-up my email? for more details on how to create new email addresses and/or mailboxes and where to find your mailbox details.
Once you have your mailbox details please open MacMail. If you have no accounts setup then you'll be taken directly to account setup. Otherwise go to Mail -> Add Account:
Enter Your Full Name, Your Full Email address and your password then click Create
(Please see How do I set-up my email? if you do not know how to access your password):
Next you should see all of your emails start to appear in your inbox.
To test your setup please go to send an email to yourself to test that it's working okay.
If you receive your test email in your inbox then congratulations, it's all working!NOTE:
If you see the following error then you need to further configure your outgoing server:
If you do see the above error then click on Edit SMTP Server
and enter the following outgoing server config information (making sure you enter your own email address in the User Name