You can get your IP from
www.abyssunderground.co.uk/ip the person with the sig that shows the IP, thats wrong as it shows NTL's transparent proxy IP for me. My script overcomes it if there is one.
Temporally install Abyss Web Server from
www.aprelium.com so you can get a page when you type in your IP (It will be a congradulations page).
Temporaly disable all firewalls and try access the webserver with the IP you got above. If it works, you can now enable the firewall and configure it again so you can see the page still. Once youve done that you can reconfigure the firewall the same way for your games. You can then uninstall the web server if you wish as you no longer need it unless you intend to run a website.
NOTE: If you have a router you need to forward ports on that too.
I run a site to do with setting up a web server at home and it is almost the same for setting up games.