A few basic tips for port forwarding
If you have a router which is connected to the internet (a lot of people do e.g all the wireless people) then your need to port forward ports 8080-8085 to your lan ip addresss in your router
Find your router on this page
http://portforward.com/routers.htm
click on any of the games does not realy matter much change the ports that the game listed to 8080:8085 tcp
(yes i know you only need to do 8080 to play games and 8085 to spec but some people all ready use 8080 for other programs so when direct hosting if it does not work frist time try port 8082 or 8083)
A common problem is to forward the ports to the wrong ip on your lan, to check what your lan ip address is
press start run then type in cmd (it might be command deepending on your windows verson)and type in ipconfig.
It should then list things like ip address subnet mask and default gateway and a few other things deepending on your windows. You want the ip address which will look somthing like your router ip address but with a dif number at the end e.g if your router was ip 192.168.1.1 then your looking for somthing with 192.168.1.*** (it's normaly 2 if you only have 1 computer)