Today, the Practice Phase began and will last until May 8. As the name suggests, this is for practice. If you're after a Murkimus or a Vanquisher title for your live realm characters, you should absolutely use this period to form teams and get ready to go, but your games won't count toward your eventual ranking or toward your title or pet.
On May 8, your rating will be reset to 0, and Ranked Ladder Phase 1 (snappy title) begins. Matches played in this period will count toward pets and titles, as well as eventual ranking. Once you're into this phase, your team will lose 150 points of rating if you take on new players, so try to make sure your team roster is sorted out before May 8. If a player leaves your team, you won't be penalized.
From May 22 to June 5, the final phase, aka Ranked Ladder Phase 2, is in action. This phase is exactly the same as Ranked Phase 1, except that no team switching is allowed.
To get yourself a Murkimus, you'll need to play 50 3v3 games between May 8 and June 5. To get a Vanquisher title, you'll need to be among the top 1,000 teams on June 5.
If you haven't registered yet and want a piece of the Arena Pass action, it's not too late! You can sign up until May 22. And if you're in the EU, you need to add 1 to all the dates above. You can entertain yourselves by doing that until the event opens tomorrow!
Lastly, don't forget that by signing up for this Arena Pass realm, you will also qualify for exclusive access to a Level 90 Arena Pass realm after you've upgraded your account to Mists of Pandaria!