Achieving True Pirate means collecting a certain amount of studs in each of the levels. Like previous Lego games this is a breeze with extras turned on. Any of the treasure multipliers will speed up the process but even without them this shouldn't be a problem if you destroy everything. If you do not get it during Story Mode then you definitely will in Freeplay with the additional areas and studs.
The multipliers come from certain red hats and are bought with the studs you find while exploring the levels. See "You may throw my hat" for more information and a location guide.