Connect the router to a computer with an ethernet cable (if it's not already connected that way). Open a web browser on that computer, type 192.168.0.1 into the address bar and hit the 'Enter' key.
(Note: As you're not actually changing anything, you probably don't need to use an ethernet cable. If your computer is only connected wirelessly try these instructions anyway).
That should take you to the router settings log-in page. You'll probably be asked for a username and password. (No, that's not the same as the password you're trying to find out!). Unless Mr Cake changed it [and most people don't] the username is 'admin' and the password is 'password'. (If it's a fairly old router is might also be '1234', or possibly 'setup').
Once you're into the settings you should be able to locate the page showing the password. Don't forget to log out when you've finished.
Chris