Howdy,
I can help you out with that one. You need to log into the the router either via airport (which is not working) or a direct connection using an cat 5 ethernet cable. Once you've established a connection, select under the apple in the top menu, under location, choose "network preferences". If your connected using an ethernet cable, under where it says "Show", select "Built-In Ethernet". If you are using your Airport to connect, you would select that. Now once in the next window, the number you want to copy is next to "Router:". That's your routers IP number. Close that window and go to Safari or what ever browser you use and type in "http://192.168.1.X. The number that appears after the "http://" is your router number you just copied. Once you hit return, you'll be into you router. When the page opens, click on the "Home Network" icon at the top. A new window will open, off to the right you see "Status at a glance", in that window click on "Edit Settings". On the next window you'll need to enter your password to change settings. This password is usually not your wireless password, but the one that you selected when you set-up the account. Try your email password if you can't remember, that sometimes it is the same. Once you get in, you'll see "Identify Network", here you can actually rename your wireless name if you want to. Below that you'll see Wireless Security. This should be enabled, next to "Authentication, THIS is where you can change to a WPA security protocol. Also, you'll need to enter a password that's at least 8 characters long. Next hit save below. That's it. Now you have to re-connect and enter the new password and you should be on.
Sorry this was so long, but didn't know how technically capable you are. Hope this did it for you.
G5 Quad Mac OS X (10.4.8) MacBook & Powerbook