Are you looking for the password to get into the router's settings or the wireless security key to be allowed to use the router to get online?
The latter is usually printed on the router's case but the access to settings password is often "password". The login name is often "admin" and both don't need the "" bit. To get to that stage of logging in to the router, you need its address.
Press your Windows key and R together then when the Run box shows up, type
cmd /k ipconfig /all
then hit the Enter key.
A Command form shows up and the information you need is beside the entry named Default Gateway.
It will look like blocks of numbers like 192.168.0.1 (thats mine) and that entry will be what you need to input to the URL bar of your browser as though you were going to an Internet address.