What you need to do is log into the router and change the settings.
From your web browser (Internet Explorer or Firefox), enter 192.168.1.1. It will ask for a username and password. Enter ';admin'; (sans quotes) for both. (If it comes up saying it can't find 192.168.1.1, try 192.168.0.1)
Once you're in the interface, click on the ';wireless'; tab. Here you can change the network name (SSID). Under the security tab (Which should be on the row of links below ';wireless), you can enter a WEP or WPA key.
I'm sorry I can't be more specific. It has been a while since I used the WRT54G.