This is one of may favorites Manga if you guy have the chance to read it. You seriously should is so big and complex and filled with imaginative drawings that makes you wonder,
The story goes like this monkey d luffy finds a pirate in his hometown,
He wants to bea pirate and ask this guy " shanks" to take him, he said no because is to dangerous for a lil boy so he gets mad. Then he eats a devil fruit that makes his body like rubber but by eating a devil fruit one can't swim.
Anyways shanks gives him a straw hat and tell him next time he sees him he can return the hat. When luffa turns 15 I think he leaves the town to later become a fearful pirate.