It all began in 1977 with the release of Episode IV: A New Hope (which at the time was simply known as "Star Wars"). That was the first movie in the real world's chronology, but the first movie within the fictional universe's chronology, Episode I: The Phantom Menace, wasn't released until 1999. Ultimately, there would be three trilogies (for a total of nine films) in the Skywalker saga, as well as various standalone movies, and TV series, and books, and comics, etc., in the Star Wars franchise as a whole. (The three main trilogies are also referred to as the "Star Wars saga," but I prefer "Skywalker saga" because, IMO, "Star Wars saga" should refer to the franchise as a whole.)

Obviously, for a long time people had no choice but to watch the films out of order, starting with Episode IV instead of Episode I. But ever since the release of the prequel trilogy, there has been some debate about the best order in which to watch the films, for anyone who hadn't already seen the original trilogy. Machete order (which was devised prior to the release of the third trilogy) would suggest skipping Episode I entirely, and watching them in the order 4-5-2-3-6. There are some really good narrative reasons for this, but since I'm old enough to have seen the original trilogy first, I'm still a 4-5-6-1-2-3 kinda guy. I don't think Episode I should be skipped, because I actually quite enjoyed it (for the most part). Still, I do think that 4-5-1-2-3-6 would be just as good (if not better) for a first-timer as either release order or episode order. Of course, whatever order each individual chooses to watch the series is entirely up to them.

Meanwhile, if you do choose to watch the prequels without having first seen the original trilogy (which I shall henceforth call the OT), I feel like there are some things you should know about the "Star Wars" fictional universe (SWU), which older fans like myself learned by watching the OT, things that it's sort of assumed viewers will already understand when they watch the prequels. But if you haven't seen the OT, some of those things might be slightly confusing for you. So... I just want to introduce you to some of the major terms and things in the SWU. (I'll link to pages on Wookieepedia, in case you want more detailed information about anything than I feel is necessary to go into, here and now.)

First of all, each of the main movies begins with the opening crawl, which itself begins with the words "A long time ago in a galaxy far, far away...." and goes on to explain what the current situation is in that galaxy. At the time of the OT, the galaxy's government was an empire (i.e., the Galactic Empire), and it was ruled by the Emperor (though he wasn't a major presence in the franchise until the third film, Episode VI). Before he came to power, the government was a Galactic Republic, which existed for about 1000 years. It was ruled by a Senate, at the head of which was the Supreme Chancellor. But there was also an independent order of people called Jedi, who used a mystical power known as the Force to provide them with various abilities, which... I don't feel the need to go into detail about at this time. But also they use lightsabers (laser swords) instead of blasters (laser guns), which most combatants use. Individuals within the Jedi Order can have different ranks: apprentices are called "padawans," though that term wasn't established until Episode I. Most Jedi who have completed their training are Jedi Knights, and the highest level are Jedi Masters. (Though I think a Jedi only needs to be at Knight level to begin training a padawan, themselves.) Anyway, the Jedi Order worked with the Republic. While they were mainly a religious order, they were also majorly badass fighters when necessary, so... they could serve in various ways both in peacetime and in war. I also need to mention that the Force has a "light" side and a "dark" side, and while the Jedi used the light side, there were others called Sith, who used the dark side. The Sith had supposedly been eliminated long before the time of "The Phantom Menace," however.

Other than that, I should say that Star Wars is a mix of science fiction and fantasy. So... you'll see lots of space battles, but also lightsaber duels (which are basically sword fights with glowy swords, though other Force-abilities can come into play, making the duels even more epic). And while most of the major characters in the movies are human, of course the galaxy has countless alien races. And droids (robots).

And... I think that pretty much gives you a good starting point. But I'll add further details if I think of anything else you need to know.