How Would Someone Become the king of the pirate? Can someone become the king of the pirates only by finding the one piece.
Be the strongest. ( Late Roger was the strongest pirate in his era at peak, Late Whitebeard was the closest to Roger's strength )
Get to the end of the grand line aka Laugh tale. (Roger was the first person to conquer the grandline so he was given nickname &quot;Pirate king&quot;.)

Also it is not a title it was just a nickname to separate him from pirates weaker than him.