By default iptables is the firewall that is enabled on linux systems. It is very simple to open ports without totally disabling the iptables service. Here are simple steps:

iptables -I INPUT -p tcp --dport <port> -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p tcp --sport  <port> -m state --state ESTABLISHED -j ACCEPT

Example to open http port – 80:

iptables -I INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT