Where can I find the WiFi password?

It is set by you in the router's configuration. You configure a router by logging into it with a web browser. This is not something you do with settings on your Apple hardware. Review your router user manual for instructions. If you have not changed the router's log-in password it may be printed on a label attached to the router. This password is not the same as the one you use when connecting to wi-fi which is what you are setting once logged into the router.