What do I need to buy to play the entire game of WoW?

You can purchase the game online and download the client it will automatically download the full game. Or you can buy the games in a store you will need World of Warcraft, World of Warcraft: The Burning Crusade, World of Warcraft: The Wrath of The Lich King and World of Warcraft: Cataclysm. You can use your trial account if you like WoW gives you he option to upgrade your account to a paying one.