You need to configure your router. I am not certain how to do this, since last time I did it was with a friend who is a network engineer. Firstly you go into your router with your web browser (check your router's manual for how to do this). Once inside I'm a bit vague on what we did, but if you look around in there you might find what you need. If I remeber correctly, we set a static IP for the machine we used as a server (otherwise the router might give it a different IP each time it is started) then we used .....ummmm what was it called... some acronym like for "demilitarized zone" [edit: DMZ -that's the one!] and set that to the static IP. You could then access the server from the web using the outside IP. To find your outside IP, search for "what's my IP" on Google.

I hope that was at least a little bit helpful.