First what you do is Open up Airport Utility, Then click in the menu "File" and go down to "Configure Other"
Then this menu will pop up
Enter into it your routers LAN IP address (for most it is 10.0.1.1) and your WPA Password (another menu will pop up and you have to enter it again)
Then Click the Button that says advanced, and another menu will pop up
Click Port Mapping and then the "+" symbol
Add what I have here and then add your LAN IP Address (Not WAN)
After that click the "Continue" button and let your airport restart, After that is done you should be able to host.
Now, don't host horrible servers.