In Mail, click on "Accounts" on the left, then click on "Add account" on the right.
On the next screen, scroll to the bottom, click "Advanced setup".
Click "Internet email".
Fill out the fields shown, substituting in your own email address and password. Please note that the "User name" is your FULL email address, not just the part before the "@".
Continue to scroll down and fill out the fields as shown.
Click "Sign in" once everything is filled out.
Your email has now been set up.
Click "Done" to complete the process.