Okay, so here's the deal. I guess I want a (VLAN?) that accepts connections from the internet. In this network computers should be able to communicate with each other, but not with the rest of the network.

I have a non-typical setup currently as follows:

Currently we have a crappy att(192.168.1.254) gateway that is connected to the internet. We forward all the traffic to our fancy router(192.168.1.1), which routes the traffic for us. I have a third router(192.168.1.3) that I would like to set up some sort of DMZ and use that as some kind of separate network. I don't know how to configure this third router so that it doesn't allow connections to the rest of network.