It's called a Security Key, not a password.
Open a web browser and log in to your router's configuration utility by typing the router's internal IP Address into the address bar and pressing <Enter>
Then go into the Wireless Settings section and the security key will be shown there.
Look in your router's setup guide for it's internal IP Address.
It is usually http://192.168.0.1 or http://192.168.2.1 depending on the brand of router.