Just like iOS’ public beta, the public beta for Apple’s operating systems were introduced a couple of years ago whereas before users had to have developer accounts in order to try it, but now the public beta means that anyone with a Mac computer will be able to sign up and take the platform for a spin.
However given its beta stage, we wouldn’t suggest that you use it as your main OS since there might be features that aren’t working properly yet, there could still be a ton of bugs, and also there could be a chance that some of your apps might not function the right way as they haven’t been updated yet.
If you don’t mind taking it for a spin though, just head on over to Apple’s Beta website and sign up and you should be able to download it. At this point in time macOS High Sierra has no set release date, but given the past releases of older macOS platforms, it should be somewhere towards the end of the year.