tl;dr- MKX is the way to go. If you can't wait, look into Ultra Street Fighter IV (the online isn't actually that bad), Skullgirls, King of Fighters, Fightcade, Dead or Alive, or MK9 to play while you wait. If you want to start getting good, watch parts 2 and 3 of this playlist as a good starting point.
MK9 does not have a big playerbase on PC, because it came out two years after the other versions. That being said, the single player is great (the only fighting game single player worth playing), so it's still worth the $20 price tag if you can't wait for:
MKX. It will have a huge playerbase, day 1 purchase for me (April 14th). This is THE game of the year, as long as there are no major launch issues (which there may be, but they should be patched quickly).
Skullgirls has a small but dedicated playerbase who will definitely help you get started. Check out Skullheart. The in game tutorials are really great, and /r/Skullgirls runs a newbie night of some description where you can play games with other newish players and get advice, but even with all of this help the game is tough as nails.
The King of Fighters XIII has a decent number of people playing, though not as many as Skullgirls, I think. King of Fighters 98 and 2002 are also available (Steam versions just came out), but I don't know what the player numbers are.
Dead or Alive: Last Round comes out at the end of the month but nobody can say how much of a community will build around it (I'd guess not much, since it won't have online functionality at launch, but you never know).
Finally, you can check out Fightcade to play older arcade games with people over the internet (like Street Fighter 3: Third Strike, Super Street Fighter II: Turbo Edition, and Vampire Savior). The games are less complicated but people have been playing them for 20 years, so it'll be tough. People will probably be very helpful, just like Skullgirls, though.
Also I should mention Melty Blood but I don't really know anything about it, just that it exists for PC and is supposed to be good (I think it came out around 2005). Guilty Gear X2: Reload and Blazblue: Continuum Shift Extend are also out on Steam, but they probably won't have too many players since there are newer versions of those games out on console.