I believe this is being asked because they released 4-6 first and then released 1-3 and now 7.
Most people say that you should watch them in the order that they came out, so you should just go ahead and do that.
That would mean watch them in this order:
Star Wars Episode IV: A New Hope
Star Wars Episode V: The Empire Strikes Back
Star Wars Episode VI: Return of the Jedi
Star Wars Episode I: The Phantom Menace
Star Wars Episode II: Attack of the Clones
Star Wars Episode III: Revenge of the Sith
Lots of people disliked the prequels (1-3), so it's up to you if you skip them, but you have to watch 4-6.