Watch:
1) A New Hope.
2) The Empire Strikes Back.
3) Return of the Jedi.
4) The Phantom Menace.
5) Attack of the Clones.
6) Revenge of the Sith.
7) and if you want to watch a non-Disney made series then there is the 2003 animated series Star Wars: Clone Wars directed by Genndy Tartakovsky.