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 192.168.8.1.
You should see a box prompting you for your username and password. Enter your username and password now. By default the username is admin, and the password is blank. Click the Login button to log in to your router.
Click the Wireless link near the left side of the page.
Click the Basic button near the top of the page.
Click the Security button near the top of the page.
When you're finished, click Save & Restart near the bottom of the screen to save your changes. And that's it! You're done!