Go into your router's configuration page (see your router's manual for how to do this, it is an address that looks something like
http://192.168.1.1). In there you want to find the wireless security section and change it to WEP security.
You will then need to enter a 64 or 128 bit WEP "key". This is your password to access the wireless network and the way you enter it is you enter a 10-character string (for 64 bit) or a 26-character string (for 128 bit) that contains only the numbers 0-9 and the letters A to F.
An example of 64 bit wep key is: 5145466d52
example of a 128 bit wep key: 3c2f2339483f2537373c57775f
Once you save your settings, your router should appear as a "secured" wireless network. When you try to connect, it will ask you for the key. Enter the same key you entered into the router.
That's it! The hardest part is finding the WEP section in the router's settings.