The first Harry Potter story in the series is:

  • In the UK and most of the world: Harry Potter and the Philosopher’s Stone.
  • In the United States: Harry Potter and the Sorcerer’s Stone (same book, different title for the U.S. market).

So when people ask “which Harry Potter is the first one,” they are referring to Harry Potter and the Philosopher’s Stone / Harry Potter and the Sorcerer’s Stone.