I'm a newbie with no knowledge of linux & little in networking.
What's the best way to learn how to setup a firewall
on a RT-N16 ?
Looks like you were not resourceful enough Google online to enrich your knowledge. Thus no amount of forum replies will ever help you. Your best bet is pay for lessons, go for a class on Linux/Unix oriented security.
go to linksysinfo.org and check tomato forums, pose any questions there, as this is a dead website full of spammers and any advice you get here is likely to be a wind-up