Yes, port 80 for web.
Before you do any port forwarding, can you get to your website (being hosted by your Mac presumably?) by opening a browser on the machine running the web service and going to
http://localhost ? If you can great, the web service is at least running and working. Now you need to get the IP of your Mac running the web service - you should be able to find this in networking prefs. Now go on to your AirPort Extreme and forward port 80 to the IP address you just got. Make sure the firewall on your Mac is open for port 80 also.
Let us know how you get on, and if it all goes bunk, tell us where this plan falls over.