You can buy specific LEGO packs for pirate ships, with all the pieces and instructions. If you want more of a challenge, you can build your own pirate ship out of other LEGO pieces and customise it. Make sure you get all the LEGO pieces you have for this challenge.
Start from the water up with the hull. Your LEGO pirate ship needs to stand up, so make the bottom wide and flat. From there, build the hull in a curved shape. It should be longer than it is wide, so plan ahead. This is like building a roof on a LEGO house, just upside down. Start each new row by putting the piece half on the last piece and working inward. Your hull doesn't have to be solid, but it makes the next step easier if it is.
Build the deck. Your pirates need a place to stand, sail and fight. Across the top of the hull, put a solid surface. If you build a solid hull, this could be a solid colour. If you build a hollow hull (to save pieces) then you will need some large flat pieces and a couple of supporting T-shaped beams. You might want to add a small barricade around the deck to keep invaders out. To really make your ship standout, build a captain's cabin at the back. It is a raised portion of the deck with a small door.
Raise the mast. Build a tall mast in the centre of the boat. You need some cross beams for a sail. The cross beams could be of a non-LEGO material to reduce the size and weight of the mast. A pipe cleaner or a piece of wire would work well. Tie a small piece of material for a sail. If you're building a large pirate ship, add smaller masts and sails.
Add the extras. This is where a LEGO pack comes in handy, but you can improvise. You'll need a flat, narrow and long piece for the plank. Finally, you need some pirates to sail your ship. Give them wire for swords.