1)Open the web browser that you use: Firefox,Google Chrome,IE,or Opera and type in the IP address for your router,and press the Enter key. http://www.routeripaddress.com/< Router ip addresses can be found here
2)A window will open and then type in your routers default username and password,and press the Enter key again http://www.routerpasswords.com/< Router usernames and passwords found here ***** Username and Password may also be located on the side or bottom of your router *****
3)Navigate to the Wireless Security\Network Security Settings Tab and open locate or change your WEP or WPA Security Passphrase. Use WPA because it provides better security.
4)Write this Security Passphrase down and put it in a safe place to access when needed.
Different in what way? The only information we can give you here depends on you being able to cable into the router. It can't be done wirelessly otherwise we might be helping you to hack someone else's system and that's illegal.
Post back the make and model of the router and we can guide you through to finding the security key. Did you get as far as logging into the router?