you need to enable ip masquarading. there are docs about it you can find by google, and lots of information on this site. i personally use an aplication called Firestarter to set it all up for me along with a decent firewall, but at the simplest level it can be done by running
/sbin/ipchains -A forward -i eth1 -j MASQ
echo 1 > /proc/sys/net/ipv4/ip_forward
where eth1 is the external interface. that's an old version of the commands though, and shouold be using "itptables" instead of "ipchains" but i can't remember the alternative version.