If you haven't played before you need to purchase all previous expansions and the original game, too.
Blizzard is offering a "Battle Chest" containing the original game and the first two expansions for a few bucks. Add the Cataclysm expansion and Mists of Pandaria to it and the monthly fee.
Maybe the "recruit a friend feature" might award you with a free upgrade to Cataclysm and/or even spare you from buying the battle chest. But for this, you need someone inviting you through "recruit a friend". I just know that a scroll of resurrection (which is for previous player that come back to the game), gives a free upgrade to Cataclysm.
The last part is speculation on my end, you have to do some research on blizzards site about this, just look for "recruit a friend". Even if you don't know someone, people playing the game get some in-game benefits from inviting other players. Shouldn't be hard to find someone inviting you.