You need to look at two things here:
1. Firewall settings with iptables - to stop unwanted traffic
2. Masq'ing with NAT - to disguise your internal network.
Try this tutorial:
Also, do a search for 'linux iptables NAT tutorial' on your fave search engine.
For setting up SAMBA, I would recommend doing this first with no firewall installed (with no internet connection) so that you can troubleshoot this before you introduce any issues with NAT / firewall.
Let me know how you get on!