This one should come naturally. Plundering ships is the fastest way to earn money and you'll need tons of it to buy all the upgrades for the Jackdaw so you'll probably have more than 30 just going for it.
If you don't, the best place would be in the north plundering Schooners and Briggs. To plunder a ship you'll have to weaken it first until you hit the red zone, approach it and hold circle .