Get the router's IP address by looking in the TCP/IP configuration panel for your wireless interface in System Prefs>>Network. Then open up a browser and type
http://<router ip address
It's normally either 192.168.0.1 or 192.168.1.1 depending on the Netgear model. You'll be prompted for a username and password. If you've never changed the admin password, it should be username = admin and password = password.
On the left hand side of the screen, there is a Wireless Setup option.
In there, you can turn off the SSID broadcast (recommended) and lock down which wireless computers are allowed to attach to your router.
Under Wireless settings, you have the option of setting up either WEP or WPA encryption, which are both supported on the Mac, but only change this if you can also connect the Mac to the router with a cable in case something goes wrong. The "Passphrase" on the Netgear is the same as the "Password" your Airport configuration on the Mac will ask for.