The Open Beta is starting tomorrow and you can already register for it on Steam right now.

To do so, visit the official store page on Steam and scroll down to the different game editions, then click "Request Access" below.
 



According to a statement on Twitter, there won't be any pre-load for this beta, so the client will only be available for download once the beta starts on September 9 at 7:00AM PT (2:00PM UTC).