You’ll be able to find all of the Marks of Fortune tied to your account by clicking the ‘Esc’ key on your keyboard. The larger UI menu should open up on top, and then click the ‘store’ tab. The store is full of various cosmetics you can purchase, and none directly affects any player’s gameplay. It only modifies a character’s appearance. On the top right, you’ll see how many Marks of Fortune you have.
You can add more Marks of Fortune to your account by clicking the ‘plus’ symbol. You’ll be able to see all of the available choices you can pick from, and they all require real-world currency to obtain. None of this is needed, though, to improve or impact your gameplay experience. You cannot use the Marks of Fortune in any other way. They only work in the New World cosmetic store.