The password to connect to a wireless router, as well as details for the admin interface where you can log in and change the network name or WiFi password, will normally be found on a label on the back or underside of the router.
If you mean you've put the SIM in a phone and are trying to connect to the wireless hotspot on that phone from another device, you'll need to go into the settings on the phone you've put the SIM in to view or change the WiFi name or password. Search for 'hotspot' in the settings.
