Go to
www.portforward.com and search for you modem manufacturer/model there.
Follow the manual and open port 23073 UDP.
You can also open 23073 TCP if you want to use server administration tools (like ARSSE and ESA).
And also 23083 TCP if you use custom maps and want people to download the ones they don't have.
One more... 23196 UDP if you wanna see your server in ASE (all seeing eye)