To fully enjoy the One Piece movies, you should start by watching the TV series up to a certain point. The movies are designed to be standalone, but they often contain spoilers for the main storyline. So, if you want to avoid ruining the surprise, make sure to watch the episodes leading up to the movie's release.
The first One Piece movie, One Piece: The Movie, takes place after episode 8 of the TV series. From there, you can follow the recommended viewing order to ensure you're watching the movies at the right time. This way, you'll get the most out of the story and its characters.