Marie had someone set up her router, and she needs to get the Wi-Fi password to set up a new printer. Leo recommends taping a piece of paper with the Wi-Fi password to the router next time so she won't forget or lose it. She can reset the router by powering the router down or unplugging it, then pressing and holding the little reset button for about 10 seconds, then when she starts it back up she can configure the router. But this would require her to set up a new password and reset all of the devices connected to it.
Since Marie has a Mac, she can actually look for the Wi-Fi password in the "Keychain" app. It has all of her passwords, including her Wi-Fi access point. She should open Keychain Access from the Utilities folder. Then she should search for the word "Airport" and she'll find a list of Airport network passwords. She'll scroll down that list and find the name of the router that she's using. If she didn't know the name of the router, she'll be able to look in the menu bar what Wi-Fi network she's connected to.