I wish to watch the series of Star Wars movies. Which is the first movie in the series?
That is a trickier question than you might think.
The first within the chronology of the Star Wars universe would be Episode I: Phantom Menace.
The first movie that was made was the one in 1977 (now titled Episode IV: A New Hope).
Episode IV: A New Hope was written in a way to introduce the audience to the universe and the characters within it.
Episode I: Phantom Menace was written in a way that assumes everyone is already familiar with the story and characters within it. It not only doesn't do as much to introduce the audience to the story, but episodes I though III contain spoilers to the later episodes.
Before Rogue One came out, the preferred way to watch the movies was called the Machete Order:
Start with Episode IV, then watch Episode V. Then go back in time to Episode II and III (skipping Episode I). Then finish with Episode VI.
However, now that Rogue One is out, that will likely change, as Rogue One is certain to always come immediately before Episode IV.
I would recommend Starting with Episode I now. But for Episodes I through III, I would recommend the fanedit versions by Q2:
Keep in mind that the Movie/TV series Clone Wars comes between Episodes II and III. And it does include important parts of the story.
If you don't want to watch a whole TV series, smudger9 has condensed the significant events of the Clone Wars series into four movies:
There is also a TV series, Star Wars Rebels, that comes between Episode III and Rogue One. To my knowledge no faneditors have dealt with this yet, but it will probably be condensed into a series of movies by future faneditors.
Also important, perhaps most important of all, is to see an original version of Episodes IV and V. George Lucas modified these films in recent years and made them significantly worse. To see an original version of these you'll want to get a reconstruction by a guy named Harmy. His most recent Episode IV is version 2.7. His most recent Episode V is version 2.0 (although 2.5 is due someday soon). Make sure you get the most recent version.
For Episode VI, I would recommend either Harmy's reconstruction of the original (I believe he is currently at version 2.5), or Q2's fanedit of the movie (which keeps the galaxy-wide celebration at the end, but otherwise removes most of Lucas' later "improvements").
So, my recommendation would be:
Episode I (Q2 fanedit)
Episode II (Q2 fanedit)
Clone Wars (preferably the smudger9 fanedits, but watch the entire movie and TV series if you like)
Episode III (Q2 fanedit)
Star Wars Rebels (ongoing TV series)
Episode IV (Harmy reconstruction of 1977 original) (and chose the original theatrical mono)
Episode V (Harmy reconstruction of 1980 original) (chose the mono again)
Episode VI (either the Q2 fanedit or the Harmy reconstruction of the 1983 original)