How do I setup a mail server on OS X Server Tiger?

I have Tiger OS X Server and I would like to setup an IMAP mail server.

someone please walk me through the steps on the server-side

Server Admin -> (server) -> Mail -> Settings -> General -> Enable IMAP

what I have to do externally?

I assume from your statement that you want to receive emails from external clients, right? (IMAP on its own doesn't do much for you)
If so, that's a whole other issue. 🙂

Do you have a domain?
Do you have a static IP address?
What kind of network connection do you have? (hint: if it's dynamic, don't bother)

You'll need to setup your firewall/router to permit incoming connections on port 25 (SMTP).
You'll need to setup your DNS to add an MX record for your domain that points to your static IP address.
If you want to access your mail when you're outside the office you'll also need to setup your firewall/router to permit incoming connections on port 143 (or implement a VPN).

The above should get you to a state where people can send email to your email address at your domain. From there you need to configure your mail client to connect to the mail server so you can retrieve your emails.

It works. Get yourself a static IP address with business-class access. Or have somebody host the mail for you.

If you want to pursue the current configuration -- and you're very far from the first to try -- you will have issues both with the dynamic address and with the port blocks. Poke around for the previous discussions. More than a few folks have gone down this path, using non-standard ports and such. (Poke around the forums for ISP and dynamic and port 25 and block and such; or for port 2525. For instance, [messageID=7619415|http://discussions.apple.com/thread.jspa?messageID=7619415], [threadID=1423245|http://discussions.apple.com/thread.jspa?threadID=1423245], or others.)

By the time you get done and get this test mostly working, you may end up with a full reinstall to un-do what was necessary to get this to work with the ISP port blocks and such.

I just want to be able to send one email from an external email account and get it to come in to my server, doesn't that all happen on the IMAP port or is that still 25?

IMAP has nothing to do with incoming mail from other servers. All other servers will send mail to your mail server on port 25 and nothing else.

Your mail server recognizes the message as addressed to a user on that system and stores the message in the user's mailbox. Your mail client then logs in (via IMAP or POP) and retrieves that message. Your retrieval of the message from your mailbox is independent of the connection (or even server) that remote servers use to send to your machine.