The beta will be sometime between now and christmas. To sign up for it, they will post on worldofwarcraft.com that they are accepting beta applications. You fill in the application, and wait. If they accept you, you will get an email in the following weeks(sometimes quite a bit longer) accepting you, and giving you the next steps.

This is the same with basically every other beta test. I should know, since I've applied to at least every good one once.