You need to have some sort of internet access, whether it be your own access through Comcast, another available wireless access point set up by someone else, etc.
The typical setup would be that you would have internet access in your home (from Comcast in your case) and some sort of wireless access point, whether it be a wireless router supplied by Comcast or one of your own (either an Apple AirPort Express or AirPort Extreme base station or a third-party wireless router) connected to their modem. This will set up wireless access for your home. Your computer equipped with an AirPort Extreme card will be able to connect to this network and receive internet access.
Otherwise, you're relying on having access from someone else.