The new beta can be downloaded from the Apple Developer Center, and once installed, subsequent betas will be available through the Software Update mechanism in the Mac App Store.
macOS High Sierra builds on the features introduced with macOS Sierra and introduces new core storage, video, and graphics technology. macOS High Sierra supports the new Apple File System (APFS) and High-Efficiency Video Coding (HEVC), plus it introduces an updated version of Metal with VR support.
Photos is gaining a new sidebar that makes it easier to access editing tools and albums, plus there are new editing options like Curves and Selective Color, and it supports external editing apps like Photoshop and Pixelmator.
There are improvements to Safari, including speed improvements and a new feature that blocks tracking data, Siri on the Mac has expanded music capabilities and a new more natural voice, and Spotlight has been updated with flight status information.
At the current time, macOS High Sierra is only available for registered developers. Apple will make a public macOS High Sierra beta available later this summer, giving public beta testers a chance to try out and test the software before it sees a public release in the fall.