What steps do I need to take?
Log in or create a Battle.net account at http://www.battle.net

Create your BattleTag, which is required to play Diablo III (What is BattleTag?)

Download the game client, install, and play between 12:01 p.m. PDT April 20 and 10:00 a.m. PDT April 23



http://us.battle.net/d3/en/blog/4963739