Ijack :
It'll be in your router configuration. Read your router manual.
not always
look at your router. on a lot of them its on a sticker on it
it should be something like
91f3lfhrh0
(thats not my wepkey ive substituded the letters and numbers)
if it dosnt work because you changed it. then you can reset it by resetting the router to factory default. refer to the manual that came with the reuter or call there support line