You can opt in for the beta at the official site. When the beta launches on May 15, players who’ve gotten in will be able to play all the way from from level one to 30. WoW game director Ion Hazzikostas tells us that the beta will be “uncharacteristically small compared to our expansions,” in part to ensure that the focus is on the proper launch this August.
Unlocking the Lightforged will require you to complete the aforementioned Argus storylines and earn the “You Are Now Prepared” achievement. In addition, you’ll have to earn Exalted status with the Army of Light. As with the Argussian Reach, this one could take a couple of weeks, as it is time-gated. However, the main story quests should help you at least reach Friendly status with the Army of Light.
FYI: If you’re returning to WoW after an extended absence prior to patch 5.0.1, it’s possible that you’ll have more than the maximum pet limit and/or more than 3 of a kind. However, this will be the only time that you’re allowed to surpass these limits. You will have to pare down your collection, eliminating your duplicates, in order to add new pets.