World of Warcraft Classic Beta begins May 15, but not many people know how to join. First, there won’t be such thing as Beta Keys. Anyone claiming such has no good intentions. So don’t fall for phishing scams.
The true way to get into World of Warcraft Classic is by opt-in via your Battle.net account page (USA). Depending what your geo location is, you may want to change the “us.battle.net” to your location’s (i.e. Europe, China, etc.)
Not a lot of people will be chosen to participate in the Classic beta. Keep that in mind. But not everyone knows how to opt-in. So your chances just grew bigger.