Help! (Setting Up Wireless)

Okay, so I'm new to the whole wireless networking thing. But I wanted to connect my PC wirelessly, so I got the Linksys WRT54G router, and the card for my PC. But I honestly have no idea how to set it up! Obviously the software that comes with it isn't compatible with my Mac. And there were no instructions in the box. Everything works fine, but I have an Ethernet connection that just plugs straight into my Mac, no modem. I am at a loss as to how to plug the cables into the router, to make it work. I tried plugging the ethernet cord into the internet thing on the router, but it didn't work. It's weird, it says it's all functioning when I do diagnostics, but then the server doesn't function, so you can't get on the internet. I'm clueless, can anyone help me?

 

Okay, well, I did end up finding an online tutorial, and tried to set up the wireless using that. However, I'm still having problems. First of all, I try to change the setting from "Disabled" to "G only", only to have it go back to disabled, every time. And secondly, I don't have a normal modem. Just a little black box thing. So I don't really know what to do with that. I did e-mail my service provider, so hopefully he will be able to tell me soon!