Simply log into the routers admin pages and select the wireless tab to view your wireless settings and password. The instructions came with the router or are available at the manufacturers web site or searching the web.
Besides read the manaul for your router, if you have a wireless gateway from Comcast (yuck), there should be a sticker on it that has the wireless settings. If they don't work, then call the gateway support folks at 800-363-2416 and have them reset it for you.
But if the router is yours, consult your manual. If the passwords or wireless settings they have been changed and you don't remember the values and can't log into the router, then follow the manual's instructions for how to reset it to factory defaults, then re-do the configuration.