http://portforward.com/
Has the best descriptions for port forwarding, it can probably explain it better than anyone here can. Just find your router model, skip the advertisement by clicking the link in the upper right corner, find "Minecraft Server" and follow those directions. If that doesn't work, then there may be something wrong with your router.. At which point you would probably benefit most from resetting the router to factory defaults and trying again, then try the same with your firewall.