Important I need a key

Also known as the Wi-Fi password.
You either use the one you created when setting up the Wi-Fi, or, if it's just a router you plugged in and turned on, it'll be printed on a sticker at the bottom of the device.
 
If you joining someone else's wireless network, whoever owns the network can provide to you.

Are you trying to connect to your own network, or at your workplace, school, etc?