On most home routers, there is a small reset button located on the back or on the bottom. Hold it in for a period of time and it will reset back to the factory settings. The initial user/password will be in your manual.
The default router username / password depends on the make / model of your router. Check the user manual. If you already set up a custom password but can't remember it, reset the router to the factory default configuration.
In many cases, routers come with default passwords. If you haven't explicitly changed it before, you may be able to search for the information online and get the password... But I'd suggest you change the password once you login for the first time for security reasons.
You will probably have to look up the manufacturer default, and reset the router to the defaults. Most router's have a "reset" button in the back. Push a pin or paperclip in there and hold it for about 30seconds with the power on. This will reset it to manufacturer defaults.