Whether you use ISDN, cable, DSL, or dial-up is kind of irrelevant. The point is, you're connecting to the internet. The gateway in turn assigns a private internet address to each of the computers you own. Usually the series 192.168.0.nnn is used. The whole procedure is automatic, once you've set it up the first time.
Many gateways build in a firewall as well.
Basically the firewall functionality blocks internet traffic that you didn't explicitly request. How well this works depends on the unit, but the fact that you have a gateway already is a step in the right direction. Configuration is done through a web page interface. A switch is inherently better and more secure than a hub because it actively makes dedicated connections, where a hub merely shares everything, like pigs at the trough. D-Link, 3Com and others make gateway products.