In order to achieve 100% completion, then you simply need to do the following:
1) Complete all levels in story mode. This is pretty straightforward enough. If you get stuck at all then simply search for guides on YouTube, or on Google. Every time you complete a level, you will also earn a Gold Brick.
2) Unlock all characters. You unlock characters simply by playing through the main story, but there are characters you will have to buy in the port level select area. These characters can only be purchased once you have beaten the right level(s) and if you have enough LEGO studs. You purchase new characters by finding them wandering about and pressing B next to them, and selecting A. Some characters, however, require you to beat up a little before you can purchase them, for example, the crew members of the Black Peal, Flying Dutchman, and Queen Anne's Revenge. Be careful not to kill them, otherwise you will have to wait for them to appear again. You will need quite a lot of LEGO studs to buy every character. Some characters require 500,000 studs to purchase, such as Davy Jones, and Blackbeard. I would personally purchase Blackbeard first, since there areas in the levels only he can access. He can be found to the far left of the area to left of the level select area of the port area, and only once you have beaten all of the On Stranger Tides levels.
3) Obtain True Pirate on all Levels. Once you have completed every level in Story Mode, you should then have the option to play through in Free Play, meaning you can select any character you like, and toggle between them. It is best to wait until you have access to Free Play mode before attempting to obtain True Pirate on all levels. Every time you earn True Pirate for a level, you earn a Gold Brick.
4) Collect all mini-kits. This can only be done once you have Free Play mode, since there areas of each level inaccessible if you do not have the right characters and these areas require characters not available in Story Mode. Whilst most can be quite easy to find, there are some particular tricky ones, and so if you get stuck, then search for guides on YouTube, etc. There are 10 mini-kits per level, and every time you collect all 10 in a level, you earn some studs and unlock a ship in the harbour in the port area, as well as a Gold Brick.
5) Collect every Gold Brick. As aforementioned, you earn Gold Brick by earning True Pirate and collecting every mini-kit in a level. You can also earn a Gold Brick by finding every item for Jack's compass. There are 8 items per level, and some are located in hidden areas you will need to be playing in Free Play mode to access. You will most likely find most of these on your own, but, once again, YouTube is your friend. Once you have done this, there will still be 5 Gold Bricks left to obtain. 4 of these can be found in the main port level select area. To the right of the level select area is the harbour, where you will find 20 ships in glass bottles. In order to sail these, you need to earn all 10 mini-kits for every level. Once you have done this, four Gold Bricks will spawn. If you go back to the level select area and then go left, there will be a pile of Gold Bricks that you can use to build something once you have collected 84 Gold Bricks. This will allow you to play a secret level. You will get the last Gold Brick by completing this secret level.
6) Collecting all 20 Red Hats. These are located in the main port area. They are hidden, however, and require a sum of LEGO studs to purchase. Please note that some of these are particularly expensive. I recommend going for the LEGO stud multipliers first, since this will make earning studs easier. If you are having trouble finding these, then simply look up a guide on either TA, YouTube, or Google.