The Fallout 76 beta test that Bethesda promised at E3 will take place in October, with participants selected from people who have preordered the game. Community admin Jurassica said in a message posted on the Bethesda forums that the test will "start small and grow over time," but confirmed that everyone who preorders "at participating retailers" (which sounds like it could open the door to exceptions, but we'll have to see see how that works out) will get in.
Naturally, there is a process involved in getting past the velvet beta rope. Details are available in the Fallout 76 FAQ but the short version is that need to first preorder the game, and then find the redemption code on your receipt or email confirmation. Take that to bethesda.net, log in (or sign up, if you haven't) and plug it in. Bethesda will contact you with the details when they're available. The beta will begin first on Xbox One, and come to other platforms later.
The PC preorder situation is a little interesting right now: Retailers listed on the preorder page include Gamestop, Best Buy, Target, Amazon, Walmart, and Bethesda.net, and it's also listed on the Windows Store. But there is currently no listing for it on Steam, which is where these things usually happen. Amazon and Gamestop preorders (and possibly others, but those are the only ones I checked) are for digital editions, so obviously you'll be downloading it from somewhere, and I certainly expect that it will come to Steam. But right now, nothing is guaranteed one way or the other, and you should probably bear that in mind if you're considering a preorder.
I've emailed Bethesda to ask if Fallout 76 will be available on Steam, and will update if I receive a reply. It comes out on November 14.