You will just need to add a rule to forward traffic destined to a port of the public interface, to the appropriate port of camera's address, where thes service listens to.
For exact instructions you should consult the manual of your firewall as the namings and administration varie, but normally you will find it under a section called Port Forwarding or Mapping.
To make things more simple, it is a good idea to disable the DHCP client from your camera (in case it is enabled), and use a static IP address for it.
