Firewall Script (Not using NAT)

I have been asked by my boss to firewall a server that we have down in a datacentre, but I am a bit lost as to where to start. I have looked on the internet for sample scripts but they all seem to be related to people running NAT based firewalls. I simply need a script that will restrict access to certain ports and services, and block everything else. Can anyone point me in the direction of some sample scripts that I can look at to learn / adapt to my use ? Thanks Whitey