If possible you should change the security method in your router to WPA2 rather than WEP. WEP is an older technology and not very secure. WPA is better and more Mac compatible. Netgear routers support WPA and WPA2 security.
If you must use WEP for some other reason then be sure that when you enter the WEP password to log into the network that you precede the password with a "$" (dollar) sign. See
AirPort Express- Difficulty joining third-party network that is using WEP and
AirPort for Mac OS X 10.3 or later- Using different types of wireless security (WEP and WPA).
To setup your computer open Network preferences in System Preferences. Select the Airport port from the list. If Status says OFF, then click on the Start button to turn on the Airport Extreme card in your computer. In the Network Name drop down menu look for the network name you use for your Netgear router's wireless network. Click on the Advanced button. In the network listing box double-click on your wireless network which will open a dialog in which you can select the type of security you have configured in the router and enter your password, then click the Add button.
Click on the TCP/IP tab, set the Configure IPv4 drop down menu to DHCP. Initially set the Configure IPv6 drop down menu to OFF. Click on the Apply button.