Welcome to the CentOS fora. Please see the recommended reading for new users linked in my signature.
To do it via the GUI become root
, start system-config-securitylevel
, check the SELinux tab to be sure it is "Enforcing", on the "Firewall Options" tab select "Other ports" near the bottom of the window, "Add", enter the port number, selecting "tcp" or "udp" as required, "OK", and accept the change after reading the warning.
On an Internet-facing server be particularly careful about staying up to date with "yum update" (preferred) or the GUI update tool.