You define aliases in /var/qmail/aliases/
For instance, if you want an alias for firstname.lastname@example.org
you would make a file called .qmail-postmaster in /var/qmail/aliases. The contents of this file would be either the user on the system you want the mail delivered too (say, root) or an e-mail address if it is a user on another system.
Of course you still need your place to store mail. Qmail uses something call maildir's, which blows the socks off mbox. There should be a program somewhere on your system called mkmaildir or something to this effect. You want to go into all your users directories and do a mkmaildir .Maildir. Then check to make sure the /var/qmail/control/defaultdelivery file is set to "./.Maildir/". Also make sure, if you haven't already, that you have the rcphosts file created so you aren't running on Open Relay. If that file is empty then put your domain in it so it knows it is allowed to deliver mail from the local machine.
Another thing to try, telnet to port 25 and type helo and make sure it responds