If you only have a modem (and not a modem/router combo) eth0 should be connected to the modem. Configure PPPoE settings and mark eth0 as external. Depending on your ISP, if you get a dynamic IP address, set eth0 as dynamic (you get an IP from your ISP) or static, if you have been given a static IP from your ISP.
eth1 should be connected as internal interface and gets a static private IP.
In DHCP module set the private scope in the same subnet as eth1 for IP addresses to be given to the clients on your network.