1. yes i know how to do that but i wont explain this here since it has been posted in the HowTo's section http://www.howtoforge.com/howtos/email/postfix
many times with all kind/type of configurations.
2. i've posted above the link for the oficial postfix documentation did you even bother to click on it ? ... there are some examples on how to configure postfix with or without virtual users
3. postfix alone will not work because you'll need an imap,pop service to interact with the postfix MTA when you're sending mail from a mail client , or webmail ... dovecot,cyrus and/or courier will do that
4. to make this easy for you guys there are some very good all in one packages out there
(smtp,pop3,imap,webmail) like iRedMail , Zimbra both are using postfix as their MTA, or Axigen Free Edition that has it's own proprietary mail system with everything.
These being said , when you don't know how something works ... read the documentation!