Luffy ate the gum gum fruit by mistake giving him the power to stretch out any part of his body. Latter he grew up wanting to become king of the pirets. Soon he made a crew of his own Roronoa Zolo who wields three swords at once and dreams to become the greatest swords man. Nami who's the navigator of the crew and used to rob pirets. They all (mostly Luffy) want to go to the grand line where they'll fine One Piece and become kind of the pirets.