See Section 3 of RFC 1918 for the IP addresses you should use on your private network. It currently handles masquerading, but the masquerading commands are expected to be pulled out into their own command in the future. Values marked in red must be changed to match your own IP address and your Internet gateway. Values marked in orange may be changed to if you don't want to use the IP address I use on my private network. Text marked in blue is used to perform IP masquerading between the private network and the outside world.
Text marked in green is used to configure the firewall.
You'll probably remove a couple of those lines if you're not running a Web server, and you'll probably need to add a whole lot more lines to let you do your thing. You'll want to keep on eye on this to see which ports you may wish to open, or to watch losers get snuffed by your firewall. Set up IP forwarding.