The wireless security key was specified by YOU (or whoever set it up originally) when the wireless router was installed.
If you don't know the key, lost it, or don't even remember having created/had it, then reset your wireless router to factory defaults and it will disable wireless security completely. Now using the user manual that came w/ your wireless router, learn how to re-enable wireless security w/ WPA2 (preferred) or WPA2, a high quality key ( http://grc.com/password ), and WRITE IT DOWN IN A SAFE PLACE so you can provide it when prompted on future wireless connections. Better yet, copy the key to a file and store it on a USB flash drive.
NOTE: To reset your wireless router, hold the reset button on the back for 30 secs (while powered on, of course) and release. Give it a couple minutes to reboot. Your user manual should tell you the IP address (usually http://192.168.1.1 but not always) and login (usually user=admin, password = admin or <blank>, but not always) you need to access its administrative interface and re-enable wireless security.