If you want to use iptables it is something like this. It depends on the setup and the flow off the traffic.
This will allow some port into the host itself.
iptables -A INPUT -i <INCOMING INTERFACE> --dport <PORT> -j ACCEPT
But iptables is not so easy to configure.
I suggest that you read this first.
Or google "iptables" for some background information.
Is it possible to give some more information about the setup and the flow off the traffic?