the first thing to do would be for YOU to watch as many films as you can, so that you are familar with the world. Everyone has their own view of the films and which order to watch them. This is the way I watch them
Episode 1 - The Phantom Menace
Episode 2 - Attack of The Clones
Episode 3 - Revenge of The Sith
Rogue One
Episode 4 - A New Hope
Episode 5 - The Empire Strikes Back
Episode 6 - Return of The Jedi
The Force Awakens
The Last Jedi
Essentially the whole story is about a boy, Anakin, who has extra ordinary powers. The Jedi are like a military or army, they seek out people with extra powers and train them to use their powers for good. Anakin gets found by the Jedi, who train him, just like any of their other recruits. However, along the way, he meets people who use their powers for bad, ie the dark side. He has to make choices as he grows from boy to man.
The films trace his journey from a slave boy, to young man, to mature man. They show the world around him, and how his actions and choices effect others. Its a story of the innocence of a young boy, became corrupted and then his redemption and legacy.
The original 3 films,
Episode 4 - A New Hope
Episode 5 - The Empire Strikes Back
Episode 6 - Return of The Jedi
are the middle part of the story
the 1990's 3 films
Episode 1 - The Phantom Menace
Episode 2 - Attack of The Clones
Episode 3 - Revenge of The Sith
are the start of the story
and the new 200's films are the end of the story and what happened next.