You can download the game client here:
but notice, that it is an MMO so you play it in the internet and if you like to play on an offizial blizzard server, youŽll need a real copy of the game to create an account.
But you can play for free an private-servers or so named freeshards.
Here is a list of wow private-servers:
to play on a freeshard youŽll have to change a file named realmlist.wtf in your wow-directory. On most websites of freeshard-servers is an tutorial how to change this file.