There are two MMOs I would recommend for beginners.
World of Warcraft and
Final Fantasy XIV: A Realm Reborn.
World of Warcraft was originally developed to be a lot more "casual-friendly" than its industry predecessor,
Everquest -- and, in that regard, it very much succeeded.
However, the game is definitely gated behind its expansions. The Starter Set includes the original game up to its third expansion, Cataclysm, for about 19.99 USD, and the most recent expansion,
Mists of Pandaria, is (I think) relatively cheap.
Don't quote me on that, though. The game is pretty easy to pick up and play, and the leveling process will help you learn how to play your chosen class(es).
The only major downside to World of Warcraft is an infamously volatile community, amplified by the inclusion of several dungeon and raid queuing tools. Finding a community/guild/consistent group to join during your time on WoW is highly recommended and, in fact, preferred instead of going it solo -- though the Cataclysm expansion pretty much made it so that leveling solo is fast enough.
Final Fantasy XIV: A Realm Reborn is probably a lot more "newbie-friendly" than World of Warcraft, but that's mostly left to personal opinion. FFXIV: ARR is the re-release of Final Fantasy XIV, made necessary by the very negative attention the original game received upon release for being a completely un-intuitive player-unfriendly experience marred by serious performance and interface issues. Technical talk aside, A Realm Reborn succeeded in delivering a true MMO experience on its re-release in August, 2013 (so the game has just hit a year old; in fact, its anniversary events are going on right now). The game has a main storyline that you can follow through, and every system the game has is explained through playing through the main story. The game's biggest feature is the "armoury system," which essentially allows you to play as any class / job following their unlocked and the acquisition of the appropriate Job soul gems.
Final Fantasy XIV has a better community, overall, but are missing some quality of life features -- some of them are due for release pretty soon in a content patch. However, like World of Warcraft, dungeon queues are also victim to a fairly hostile community. It's difficult to tell in the game who is new and who is not, because a single character can play as all of the game's classes if they unlocked them; so for a majority of dungeons, a lot of veteran players will be assuming that their party members know what they're doing and
will react negatively when someone doesn't.
World of Warcraft's Battle Chest, which includes everything up to Cataclysm, is
19.99 USD.
Mists of Pandaria is (last time I checked) on sale for
9.99 USD.
Subscription costs
14.99 USD per month.
Final Fantasy XIV: A Realm Reborn costs
24,99 USD and is available on Steam.
Subscription for an ARR "Basic" account (only allowed to create one character on eight different servers) is
12.99 USD (anyone can correct me).
A full subscription costs
14.99 USD per month.