To set the Security key you need to have access to your Modem/Router.
I assume that you know the username and passsword to access your router.
Follow the steps below.
1) Goto Command prompt and write
now you will see the ip address of Default gateway which should be like this 192.168.x.x.
2) open internet explorer and type this address in address bar and press enter then it will ask u about username
and password. Provide it and if you dont know then try these which are common
if still not connected then ask your service provider for help. If you are connected then follow the next step.
3) Now look for Wireless settings on the left and then click on it.
4) now you will see the options on the right. At the bottom you will see Network Key (8 ~ 63 characters).
just type the key here and apply it.
Reboot your router ..
Now test it.. Best of luck.