The username and password for a mailbox are set when a mailbox is setup. As a general rule, the username is the whole email address.
The password is whatever was entered when the mailbox was setup. If you forget your password you will need to reset it in your control panel. Passwords are encrypted after being entered and cannot be looked up, they will need to be reset.
Mail servers, both incoming and outgoing, are mail.yourdomainhere.com. (Replace yourdomainhere.com with your website domain.)
You can also refer to your hosting setup letter if you are unsure what mailservers you should be using.