|If you're talking about logging into the router's management interface and you've forgot that password, then what RMT2 said applies.|
If however you're talking about the password (key) that you use to connect a wireless client to your WLAN (wireless LAN) then you need to login to the router's management interface to find it.
Once there, go to the wireless setup page. Some routers show the key for the wireless network in plain text. Others show only stars but have an option (typically a box you can check) to show the password in plain text. Others show stars and there is no way to show the password in plain text.
If you have the last kind, you'ld have to redo the password and then use the new one the next time you login with any wireless clients presently connected to your WLAN. The new client would of course have to have the (new) password entered too.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.