If it isn't on a sticker on your router, and it usually is if it was provided to you by your ISP. Then you will have to log into your router with a computer that is connected to it (wifi or ethernet) and log in and find the password within the settings. You can ask a new question to get a detailed procedure of this if you need to or just search for the instructions. Remember to give your router make and model so we can be as specific as possible with instructions.
If you mean the WiFi pass phrase, the definitive place is in the wireless security configuration page. Log on to the router and go to the appropriate configuration section.
If the pass phrase is still at the factory default setting, it will be in the manual or on the router's label. If the router was installed with security in mind, the WiFi pass phrase and the login password to configure the router will both have been changed when the router was installed.
I hope this helps.
its on a sticker usually on the bottom of the router (underneath the router, turn it upside down)
Below the router while colour sticker.