Two factor authentication is only available on certain email plans. If you are not certain if your account has access to the 2FA feature please ask your domain administrator.
If this is available, you should see an option under Preferences > Accounts > Account Security, called Setup two-step authentication.
Note: Please keep in mind that some mobile email clients do NOT support 2FA and in order to be able to continue using these devices you will need to use the Application passcode feature and enter this code as your password.