You need to assign a new ip address to the card. In this case the address of eth1 must be on a different subnet to eth0. Configure the card connected to the adsl router on that subnet eg 192.168.1.0/24 use 192.168.1.2
If you want to share internet connection then you should install a basic firewall using iptables and then implement ip-masquerading. You can find good howto's on this here
. Remember that the gateway for the server box is the ip address of the router and the gateway for boxes connected to the server is the ip address of the card they are connected to.
You now need to restart the network services which are normally in /etc/init.d in Debian the command is /etc/init.d/networking restart.