The default value can be found either at the back of the WiFi router or in the manual that came with it... this can be easily found if you're referring to a home WiFi router; if that is an office WiFi router, you need to contact IT.
If you have access to a different device that is already connected to the same WiFi router, you could check the value that was used in that device. For example, if a Windows 7 PC is connected:
bring up the list of available WiFi routers by clicking on the WiFi icon in the task bar,
right-click on the WiFi router you want to connect to, click on "Properties",
go to the "Security" tab and
select the "Show characters" under the "Network security key".