because each router is different, the best tutorial for you will be here : http://portforward.com/
Find the manufacturer and model on your router and select it in the list
An advertisement page will open, just click "Skip this advertisement" on the top right corner of the window
Then choose default guide or a game to add, because gameranger is not always listed there... and open the UDP 16000 port instead of the port described for the game you chose.
Reboot your router if possible, it can help.
Optionally, you can test if it works with their tool http://portforward.com/help/portcheck.htm, but just launch gameranger and host a room, it's the final test ^^
The best defence is attack!