You find the scrap of paper where you wrote it down when you set up your router.
If you lost that, you log into your router and set up a new one. If you lost your router login you reset your router to factory defaults, and set it up again. This time, keep the scrap of paper in a known location.
Or, if you are trying to leach, you could just stop leaching off other people's wifi.--
My place : »www.schettino.us