You need to access the routers configuration via web browser, normally 192.168.1.1 and it will ask for username/password. If you haven't changed this from default, it will be on the label on the backside of router. One in, change the default password as a big security risk.
Now look for an option to port forward, put in the IP address for the xbox, which should have been set a static IP in the the range 192.168.1.x with x=2-254.
Then select the port ranges for the game and enable.
Saying that, if the router suport UPNP, normally not needed, check that it is enabled. Still worth setting a static IP for the xbox.
Guides for most routers from www.portforward.com