I got a OS X 10.5 standerd server set up... how do i use it

The documentation that came with your purchase of Mac OS X Server should provide some help with configuring your FTP server.

Be aware, however, if you have Verizon residential service rather than a commercial rate plan, your agreement will explicitly forbid operating an enterprise-grade FTP server on a dynamically assigned IP. If they note a large amount of outbound FTP traffic, they can and will suspend or terminate your service.

Uh..

your external ip, as long as you have mapped port 21 to the server's internal ip.