The password key

usually by using its IP address.

you will need to use a computer that is already connected to the router (internet)

open up internet explorer (or other browser) and in the address bar try the following:
http://192.168.1.1
http://192.168.0.1
http://192.168.2.1
http://172.16.1.1
http://172.16.0.1
http://172.16.2.1
also, i see someone recommends trying
http://192.168.1.254

it will ask your for a username and password which, unless changed during setup will be the router's default such as
admin/admin
admin/1234
etc

both the router's default IP address and default username and password will be in the instruction manual