Cisco 2801 w/ 3 Interfaces

I have a Cisco 2801 with an HWIC-4ESW installed in slot 1.
I have an internal network, an Internet connection and a seperate LAN connection that will go to another building. The two LAN interfaces will plug into switches and the Internet interface will go to a router provided by the ISP.
So far, I have been able to get interface F0/0 to connect to the Internet and interface F0/1 to be the internal interface. I’m trying to get the HWIC to be the interface to the other building but I can’t figure out how. When I add the HWIC, I get four new sub-interfaces off F0/1 (F0/1/0, F0/1/1, etc) and a new Vlan1 interface.
Am I going about this correctly? Can I accomplish my goals with the hardware I have? If so, how?