Connect your computer to a free Ethernet jack on the router. Make sure that you plug your computer into a port that's grouped near several other Ethernet jacks --- the isolated Ethernet jack labeled "WAN" does not work for password recovery.
Type in your router's address -- this is some variation on "192.168.0.1" -- and log into the router's administration panel. Refer to your router's manual if you don't have the password or login address available.
Click the "Wireless" tab to access your router's wireless settings.
Scroll down the page until you find the router's network settings. The password is typically listed near a pull-down menu with wireless password settings; this menu contains settings such as "WEP and "WPA."