Hi:
First thing to do is break out the instructions for the Netgear. Look for the option to "reset" the router. Normally the reset is a small hole that can be accessed by extending a paper clip, inserting it, holding it down for about 10 seconds. After which, remove power from the Netgear for about 30 seconds or more. Plug it back in. The Netgear will be reset to factory settings.
Now you should be able to access the Netgear from your browser (Safari) and log into its firmware menu. Be sure to read the instruction manual. Unfortunately, Netgear does not give support for Mac systems - they will only help you out if you have Windows. And, most ISPs are not up to date on Mac computers having for the most part received their training on Windows systems.
Connect your DSL modem to the Netgear (per your ISP instructions) and then by CAT-5 cable from the router to your Mac. Once you get your machine working OK with the router, you can get back into it and turn on the wireless option. Afterward, once wireless is working OK, you will no longer need to attach the CAT-5 cable.
It's recommended that you use security (WPA, WEP, etc) which means that whatever password or hex code you enter into the router, you will have to do likewise for your Mac and the X-Box. It all sounds complicated but it's not really. If you feel you don't understand how to set everything up, I suggest you ask a friend or someone who is knowledgeable to assist.
Regards.