Standard router address on your network should be 192.168.0.1, if that doesn't work try 192.168.1.1 (if neither work you'll need to check your instruction manual). Type this into the address bar of your browser.
If it does work, you should get a prompt for a username/password. If you haven't changed it, you might be lucky and it'll be admin/password
Assuming you're luck and you've made it this far. Look through the menu items on your router for port forwarding/triggering or something along those lines. At the same time, go to start -> run and type cmd. In the console window type ipconfig, you'll get your IP address. Now, back in the browser you'll need to make a new entry, type in the ip address you got from the command prompt and the port (I have no idea which port it is, you'll need to google that).
What you're basically doing is allowing network traffic to go through that port on the router and access your computer. Which means that people on your LAN can connect to your game.