Earlier today, Apple seeded new iterations of the second iOS 11 and tvOS 11 betas to developers; now, those builds are finally available to the public. The updates were announced at WWDC earlier this month, and the firm has kept to its promise of a public beta before the end of the month.
Of the two, iOS 11 is certainly more feature-heavy. It contains an all-new Control Center, notifications work differently, drag and drop, and there's a new Files app. There are also a number of iPad-specific improvements, such as a new dock that allows you to fit many more apps on it, and a new way of implementing Slideover and Split View. In fact, you can now combine Split View and Slideover to show three apps on the screen at once.
There are also some photo editing improvements. You can now trim live photos, and when you take a screenshot, you'll be able to tap a thumbnail to go to a screen where you can crop it or write on it.
tvOS 11 is much less significant of an upgrade; in fact, it wasn't even worthy of being shown off at the main keynote. There really won't be any new features that you'll notice.
You can sign up for Apple's Public Beta Program here, or if it's still too early for you, you can wait a couple of weeks until the next build is out. The updates will be generally available this fall, likely at the end of September.
Finally, a macOS 10.13 High Sierra public beta is still on its way, but it's listed as "coming soon".