So I have come to this glorious thread for some help. Out of what seems like the blue, Blizzard just flat gave me a battlechest at some point. So I own WoW up to Lich King, but I've never played. I decided to give the free trial a go and I think I might take the plunge. Any sort of advice for someone coming in brand new to the game? Also would it be beneficial to upgrade through Pandaria from the get go, or can I wait a bit on the latter two expansions?
I agree with Berordn, choosing a class is very important. Healing and Tanking are a lot of responsibility for a new player - DPS will be the easiest. Figure out if you want to do melee or ranged damage, and study the unique aspects of each class. Race isn't as important as class but make sure to choose someone you won't mind looking at for several hours.
It would help a lot to have an IRL friend to guide you through the game, as it is quite complex, but luckily 2013 WoW is very user-friendly to new players so you shouldn't have a problem learning the basics on your own.
Addons are a big part of the game - user created mods that can enhance gameplay (addons are mostly UI changes/additions). I use Mappy for a bigger minimap, Titan Panel for top and bottom borders that display information like FPS, gold, coordinates, time, repair etc. Auctioneer (particularly the appraiser) is essential for using the Auction House.
Basically just do your research and jump in. This game is great because you get from it what you deserve - i.e. put in the time and effort and you will be rewarded.
Online resources are hugely important for this game. Wowhead will be your main source of info on items and NPCs, especially the user comments area. Wowpedia is great for quest and zone info. Noxxic or Icy Veins are where you can find class builds, spell rotation, stat priority, basically how to play your character.
Good luck and have fun!