Presumably you are looking for the wifi encryption key. The default key may be printed on the router label along with the default SSID. If you haven't changed it, then it should work. If it is not, there are other options.
With an administrator account on Windows computer connected by wifi, you can retrieve an wifi security key by navigating to Control Panel > Network and Sharing Centre > Manage Wireless Networks. Double-click the SSID and click the Security tab; there's a tick box which displays the key characters.
Otherwise you may need to reset the router and reprovision.