yes, apologies for the not clear clarification, I meant attack against other players.
To capture a neutral port, you have to sail in the closest freetown (if your nation does not own a single port) or your nation port. Once there, go to the quest window (Q key) and select "Conquest tab" and pick up a mission, sail to the said port within the attack timer and join the mission. Once the battle is won, Port Battle is scheduled