Clueless
Oh dear.
You have asked for a drink from the fire hose friend.
If I had to take a stab at this... probably the two games in current circulation which have the most support (though all editions are still around and kicking, but I'm picking these for accessibility) are going to be two games from two different companies.
The first game is what the community calls fifth edition, or "5e". However, they're currently just calling this "Dungeons and Dragons" on the cover of the book and in the official catalog, which makes it hard for someone new like you to know what they're getting.
I sorely wish I could link you to the right books/site so I could be clear what books are fifth edition, however I did an experiment and typed "5e player's handbook" into Google and the first thing I got was an Amazon link to the right book, so that's my workaround.
Anyway, this is the official ruleset, the most recent one and a game that's going strong because it's the current one.
Now, another game that's spiritually a successor to Dungeons and Dragons that's basically a permutation on an older rule set that's still insanely popular and is still supported by its publisher (Paizo) is called Pathfinder. I don't have to explain this one because Pathfinder is Pathfinder and you can't accidentally order the wrong edition of it XD
I know, this is confusing as all get out. But you are dealing with a monster here and I've got to give you some toe hold on it. You can figure out other stuff later.
I am not going to get into the differences between these two games because they are many and if you are clueless it won't mean anything to you at this point to explain.
Now, what would I do if I were you?
I probably wouldn't buy a game just yet. I'd try to figure out what kind of settings are out there that you like. What got me into Dungeons and Dragons was I really liked the world of Dragonlance as a kid and it was the novels that got me into the game.
So, probably the first thing to ask is, what kind of world or aesthetic or genre are you into? Do you want classic Tolkien Lord of the Rings stuff, or do you like pirates, or wacky steampunk? Fantasy is a very wide genre.
Naming some particular novels, books, shows, movies etc. will probably give people a way to tell you "Well if you want a game like X, try Y."
I think if you nail down a particular... aesthetic, for lack of a better word, it'd be easier to give you advice. I wouldn't recommend Eberron to somebody who really wants to play in the world of The Hobbit for example.
If you have no idea, get the Enhanced Edition of Baldur's Gate and play it. It's an old school computer game that any system can run that was updated to be easy to run on modern computers a while back, it's not terribly expensive, any computer can run it because it's such an old game, and best of all it uses the second edition rules in Forgotten Realms and it's a wonderful introduction to a lot of concepts without being so jarring. That said... it's also going to be quite an education for a true novice, but if you can figure out how to play that game it will be a huge leg up on the pen and paper version.
Then, eventually, I'd probably buy a Fifth Edition Player's Handbook, a Pathfinder Core Rulebook, and, this last one is FREE, I'd get Swords and Wizardry (which is a clone of super old school Dungeons and Dragons) and I'd just read them all, cover to cover. If money is an issue, you can recoup part of the cost of a physical book selling it after, or try to find a used one. They won't make a lot of sense to you, but you'll probably like one of those three better than the rest.
If Fifth Edition is your favorite, you'll probably like that best and you'll probably like Second Edition too.
If Pathfinder is your favorite (like mine), you probably like the third edition and "3.75" edition best (the latter being the nickname for Pathfinder).
If you like the Swords and Wizardry book, then you're really old school and you probably like First Edition, Basic, and maybe some second edition type stuff the best.
(And I haven't mentioned 4E not because it's a bad game just because it's... a weirdo, it's the least Dungeons and Dragons like game to ever bear the name).
Anyway, that's the best I've got. Again, a drink from the firehose.
