firewall / ipchains ?

[afj]

another wee question,   i have 2 (physically seperate for security resons) networks. i'd like to be able to access both from a linux box. the idea is that the linux box will have 2 nics and a firewall to stop any machine other than the linux box itself from accessing the 2nd network.   I don't know firewalls at all - i think i need to use ipchains ? Any pointers to documentation more for beginners than the man page ?   Thanks in advance,   afj