Open a web browser like internet explorer or Netscape. Enter the ip address of your router in the address bar of your browser. In the picture above the address bar has http://www.google.com in it. Just replace all of that with the ip address of your router. By default the ip address should be set to 192.168.1.254.
You should see a box prompting you for your username and password. Enter your username and password now. By default both the username and password are admin. Click the Ok button to log in to your router.
Click the Configuration button near the left side of the page.
You should now see a new menu. In this new menu, click LAN.
You should now see a new menu. In this new menu, click Wireless.
Click the Wireless Security button near the left side of the page.
When you're finished, click Save Config to FLASH near the left side of the screen to save your changes. And that's it! You're done!