If you have opted in to beta testing World of Warcraft Classic via your account management page, you may want to check the BattleNet launcher to see if you have been selected. The beta servers are currently open and taking players in to play the highly-anticipated reboot of "vanilla" WoW.
Go to the BattleNet World of Warcraft page and down under the video on the left side under Region/Account, click on the drop down menu and click on Beta: Classic. If you are selected, you will see the Install button. (Yes...yes, I've been chosen!)
For those who might not have been selected, you can check out Twitch where Classic is trending.
For those who haven't yet opted in, head to your Beta Profile Settings page and tick the box next to "World of Warcraft Classic" to become a potential tester.