[Exim] Exim as a central hub.

I'm needing to install exim in a way I've not done before, and I am
looking for some info to help. I am getting this whole system switched
over to exim, need to start at the central server for now.

    INTERNET
       |
    external mail server
       |
      EXIM
       |
    internal mail server

I am not sure how I would tell exim to route the messages. Can someone
point out what I need to look at?

Derrick MacPherson
derrick@???