there are many a good tutorial out there to look at man...
I think you need to understand what is meant by a "default policy"...
you should be running a "DROP" by default policy... then you only make rules for connections you want to come into the box.... everything else gets dropped...
I would check out the FAQ post listed at the top of this forum as a start... then if you are serious about learning linux firewalls (and more specifically, iptables) I would pick up Robert Ziegler's book "Linus Firewalls". I have found it to be a great reference/asset.
One thing you will need to look into is masquerading since you are firewalling a dial-up connection...
GL yo...
