Check the router's documentation, or the router itself. You may find a label on the back or bottom of the router, or maybe inside a back-up battery compartment. Look for a long number that may be labelled as a pass code or password. Write down that number.
Go back to the set-up screen for your printer. I suspect that you'll likely have the option to connect to the router either automatically, using the WPS button, or manually by entering the pass code or password on your keyboard.
If you're still having problems please let us know what brand and operating system your computer is and what kind of router you're using.
