You need to go into the routers internal settings to see and change the security key.
Open your browser and enter the routers address , it may be printed on the bottom of the router or on a sticker on the side. You may also find it in the user manual along with the default user name and password.
Enter that router address in the address bar and hit enter , a log in window will open and you can put the default user name and password , unless you have changed it.
Once in the routers settings go to the wireless section and the wireless security sub-menu and you will see where you can enter a word or passphrase to be encrypted and that is also where you'll find the wireless security key.
If you want to change it you can use one of the encryption methods (WEP, WPA or WPA2) , with WPA ans WPA2 being the stronger encryption methods and will give the stronger security key.