to find your router's ip address (assuming you have one), start up a command window (start->run->cmd) and type in "ipconfig". Look for "Default Gateway". That's your router.

Many consumer routers use 192.168.0.1 or 192.168.1.1 as a default, but this is completely arbitrary. Anything in the 192.168.x.x, 10.x.x.x or 172.16-31.x.x range would indicate that you're behind a router, and you should be able to configure it. Anything outside of those ranges indicates that you've got a public ip address and aren't behind a NAT router (ie: maybe directly plugged into a hub/switch or a cable/dsl modem).

Of course, it's distinctly possible that whoever set it up changed it to something in that range, and set a password on it. Talk to the people who you live with, if any of them might have that info or be willing to just set it up for you.