How can others connect to port 80 on my computer via an IP address?
Within the Airport Utility, you have an option for Port Forwarding (or even setting up a DMZ Host if you want all unsolicited incoming traffic to go to your web server, however this is unadvised for general security as it is best to only allow what you need to reduce attack vectors).

Using this option, you can set up a port mapping between port 80 on your external interface to port 80 on the NAT IP address of your internal web server.

This will provide immediate access to your server over port 80 to the outside world.

That however assumes that you already have a web server properly configured to deliver data over port 80.

A good first step there would be to use the Web Sharing setting under "System Perferences -> Sharing" to see whether this meets your needs. It will also enable you to do an immediate test to make sure that everything is working.

If you need something a little more comprehensive than what you are provided within the OS X client, look at OS X Server ($50 on the App Store) or MAMP (free and paid offerings).