firewall setup fails in suse7.1

hi list, I'm new to linux, upon bootup before the gui login( whats that called in linux? anyway),i get a message saying the firewall script needs to know the external internet interface. How do i fix this in simple terms. I don't have any luck figuring out what the manuals are trying to say. I have suse7.1 , behind a dlink 704p router & ADSL. Unfortunately I am still a gui moron, I have been through yast 2 many times and have been able to enable most of the items the manuals mention, but i still can't find how to tell the firewall script the external internet interface. tyia