You need to connect to it by cable for this exercise. Being a Belkin, the router's IP address will almost certainly be 192.168.2.1. Type that into the URL bar of Internet Explorer as though it was a website, then press Enter. You'll be faced with the log in page and the login name is usually admin with a blank password - hopefully no-one has set one. Look for Wireless and then Security in the left hand pane and the key you need is in there somewhere.
The reason for being wired to it is to prevent hackers accessing other peoples' routers.