I don't know why nobody tells you this basic stuff. Actually, I do know why. You are at the boundaries of two and more manufacturers so you need to know a few fundamentals (which I learned the hard way).

Your cable modem or DSL modem may support more than one computer, by wire, and also wirelessly. If it is wire only, you understand that you can support only four devices because there are only four ethernet ports. If it includes wireless, you might not understand that you are still limited to four devices. Pick up a couple more devices, computers, wireless printer, iPads, iPhone, Wii, Kindle, and soon you have some baffling problems. The DSL modem should release any device which hasn't spoken to it lately. Good luck! Some devices just seem to hang on, and you can't figure out why some of your devices are not getting their wireless. My wireless printer seemed to hang on even when it was turned off, so I had to unplug it to free up the channel. You can avoid these mysteries if you stop using the wireless with your DSL or cable modem.

So you need something like Apple Airport, or Apple Airport Extreme. The latter has greater range. These support a very large number of devices, many, many more than I have.

If your Apple computer is up to date on software, throw away the CD that comes with the Airport.

Plug an ethernet cable from the modem to your new Airport.

Make yourself familiar with Preferences--->Network,
Make yourself familiar with Applications--->Utilities-->Airport Utility
The explanations there are pretty clear except for one thing I'll tell you next.

I found everything seem to work perfectly, but my new Airport blinked yellow (not green) and I could not finish the installation process. What you need to do to finish up is connect your computer by wire to the Airport. They don't want anybody doing critical stuff (like setting passwords) wirelessly to your Airport. Think of the security issues.

After you have finished the airport setup, you can remove the wire, until you want to change something, which may be never, or may be some time soon like maybe when Apple suggests an update to the Airport software (so you may as well leave the wire connected).

I hope everything I've said has been correct. I hope it's been helpful to you. If it has, please give a dollar next time you see a street musician.