That's defined in your router.
There might be a default one (study the manual) or you might have changed it yourself to something that's different from the default. Then change it again if it doesn't show on the router management page. How to reach the router management page via your browser is in the manual also.
I am attempting to add a laptop with Windows 8 to my home network but I do now have the Network Security Key that it keeps asking for. I am running Windows 7 64bit. How can I locate this key? Thank you