Tutorial

First we are going to configure your CentOS system to automatically start the NetworkManager on startup. You can do this by running: 

chkconfig NetworkManager on

Then we are going to start it so we can use it right away without needing to reboot: 

service NetworkManager start

If you have done that in your graphical environment (Gnome), your Notification Area (usually in the Gnome panel on the top-right) will show a new icon. If you left-click on this icon, you will see a list of possible Wireless networks to connect to.

You may also want to disable the network and wpa_supplicant services at boot time, as NetworkManager will now handle these. For this, simply do:

chkconfig network off
chkconfig wpa_supplicant off