Hundreds of millions of people are born between the release of each Star Wars movie, and millions of those people will eventually watch all eight films (and counting). There is a robust debate among Star Wars fans about the correct order to watch the movies if you've never seen any of them. This debate is complicated by Rogue One, the new Star Wars movie that takes place just before the original Star Wars (Episode IV) but stands outside of the triple trilogy saga. Where does it fit in if you're watching Star Wars for the first time?
George Lucas, the creator of Star Wars, urges people to watch the movies in chronological order, starting with Episode I: The Phantom Menace, which was actually the fourth movie made. Presumably he would stick Rogue One in between Episodes III and IV. "Start with I — that's the way to do it right," Lucas told Vulture: "1, 2, 3, 4, 5, 6. That's the way they're supposed to be done. Just because it took a long time to film it doesn't mean you don't do it in order."
This is a minority opinion. Some fans think the movies should be watched release order: IV, V, VI, I, II, III, VII, Rogue One. Computer programmer Rod Hilton, on his blog Absolutely No Machete Juggling, came up with a better viewing order for Star Wars newcomers, known as the Machete List. He advocates a hybrid viewing order that focuses on Luke Skywalker: Episodes IV, V, I (optional, discouraged), II, III, VI, VII. As for Rogue One (and the upcoming Han Solo movie and any other tangential Star Wars films), Hilton is agnostic. They "can all pretty much be viewed in any order, as supplemental material outside of the 'main saga,'" he says. "Before watching any of those movies, you just say 'this takes place between X and Y' and you're done."
You can read stronger opinions on where Rogue One fits in the viewing order at Reddit and Star Wars fan sites. The obvious places to insert the new movie are first (as an intro to Episode IV: A New Hope), or fourth (between Episode III: Revenge of the Sith and A New Hope), or last, as a standalone movie, or in release order. In other words, if you and your newbie child/friend already have tickets for Rogue One, enjoy: It's supposed to be a good one.