Does your computer connect to the internet through Wi-Fi? Or wired with ethernet?

If your computer has been connected through Wi-Fi in the past, then you can look up its settings:
  • The old defaults may be be stickered on (the bottom/back of) the wireless router/modem/“home hub”. (I changed it on my modem, for personalization and security.)
  • The last used network login settings will be listed in Keychain Access. Use “AirPort network password” in the search field to filter on that kind of login. Sort by date to see the last ones used on top.