Connect your computer to any one of the four LAN ports of the router using a ethernet cable.
Open an Internet explorer and type the router IP address which would be either
http://192.168.0.1 or
http://192.168.1.1 in the address bar and press Enter.
The Login window will be prompting for a Username and Password. The default Username is admin and the default Password is password
Click Wireless Settings under Setup menu.
Under Security Options, select WPA-PSK (Wi-Fi Protected Access Pre-Shared Key)
Enter a passphrase in the passphrase box. The Passphrase must be over 8 characters in length.
Click Apply
Then dissconnect the ethernet cable and browse for your wireless network and try to connect to it and it should prompt you to enter the passphrase which is the same as you entered into the router setup screen above.