Beginning May 15, players around the world will be able to return to Tristram to discover the dark secrets of a fallen star and begin their quest to save Sanctuary from the impending demonic invasion. To secure your place as a barbarian, monk, demon hunter, witch doctor, or wizard right now, simply log in to your Battle.net account, and then purchase and download* Diablo III, and you’ll be ready to play when the game launches.
Source: Battle.net