You'll need to install the Battle.net launcher which is like Steam but just for ActiVision Blizzard's games. You can play for free up to a certain point (character level 20) but then you will need to buy into the monthly subscription.
https://worldofwarcraft.blizzard.com/en-us/startThere is 2 main versions of the game. The modern version (referred to as "Retail") and the retro version (referred to as "Classic"). Retail is what I would recommend to a new player, just to get a feel for if you'd enjoy the game. I also think Classic might require the subscription to play, but I can't remember. Basically, classic is an older version of the game when there was a lot less features because people enjoy it more. It's hold your hand a lot less than Retail does. Don't worry about player count, as there is a high volume on both versions.