open up internet explorer, netscape, firefox, whatever internet browser you use, and in the URL bar (where you type in websites) type in
192.168.1.1
if that doesn't work, then change the second to last number, putting it up by one each time, like so
192.168.1.1
192.168.2.1
192.168.3.1
192.168.4.1
etc
until you are prompted for a username and password.
Some ISPs will provide you with a username and password for the router, however the vast majority keep it at stock settings. I will give the username and password for different brands of routers, but if it doesn't work, then post up your routers brand and model (usually has the model on a sticker on the back).
Netgear:
Username = admin
Password = password
Linksys:
Username =
Password = admin
Belkin:
Username = admin
Password = admin
I am sure I am forgetting several brands, but can't remember right now. Usually it is a combination of admin, password or blanks for the username and password. If you want to search for it your self, type in "<router manufacturer> default username and password" (without quotation marks) and it should come up with it on one site or another. For example, to get the belkin one, I typed in (again, without quotation marks) "Belkin default username and password"
Once you are in, try to find where it says broadcast SSID, or broadcast access point, or something similar, and make sure it is checked to yes, or accept, or on, or whatever the possitive setting is, then hit apply. See if your xbox can see it.
If it can, try to connect. If you are prompted for a passphrase, password, code, anything along those lines, go back to the router settings (follow the steps from the start if youforgot how or don't know what I mean

) and go to wireless settings or internet security until you find security. It will have either WPA, WEP, some kind of abbreviation followed by a code. It will probably also have 64bit and 128bit mentioned nearby aswell. That is the code that you need.
Try to connect again, putting in the code when prompted.
If it still can't connect, look on the xbox adapter and look for a label saying "MAC address" (without quotations). Make a note of this.
Again, go back to the router and again, go to either wireless settings or security and look for access list. It may have its own area. There will be an option to add a device to the access list.
Do so and for device name, it is something so you can remember what it is, so call it xbox, or <son's name>'s xbox or something along those lines, so if you need to alter anything in the future, you know what that is. You will be prompted for the MAC address. Put that in, and you may be prompted for an IP. If that is the case, it is often for a static IP in the router. This just means so it is asigned the same IP address in the network. Generally it will have 192.168.1. filled in, and you just have to put a last number in. Fill that in if asked for.
Again, try to connect your xbox. If it still isn't connecting, post back and tell us any messages coming up on the xbox, or where it stops when you test connection