In the picture above the address bar has http://www.google.com in it. Just replace all of that with the internal IP address of your router. By default the IP address should be set to 192.168.1.254.
Click the Wireless button near the top of the page.
In the box called SSID put a name that you make up. You will need to remember this name because it will be used to connect to your wireless network. Do not make it obvious to guess, the network name should be difficult for your neighbors to guess. If you can not think of anything, simply use your first name.
In the box called Channel No., pick a random channel number. Channels make it so that you and your neighbors can use your own wireless networks without interfering with each other. By picking a random channel you reduce the chance of your neighbor using the same channel as you.
Click the Configure WEP button near the center of the page.
Select 128 Bit from the WEP Data Encryption drop down box.
Select Open System from the Authentication Type drop down box.
Put a dot into the Key 1 radio button.
Now put the key you made up in the box called Key 1.
When you are finished, click the Apply button.
When you're finished, click Save near the bottom of the screen to save your changes.
And that's it! You're done!