iOS 12 has been been in beta release for members of Apple's paying developer community for since their annual developer event WWDC. This morning, they opened the flood gates on their public beta program. So, if you've got the courage, you can install a beta release of iOS 12 to your iPhone or iPad or tvOS 12 to your Apple TV.
Access to the beta versions is through Apple's beta software program. You'll need to sign up with an Apple ID - your iTunes or App Store credentials will do the trick. That will let you install a profile to your phone that will hook you up to the beta programs and the update will hit your phone through Apple's usual over-the-air update processes.
I repeat the same cautions I offered when Apple released the first beta versions to developers. Betas are not stable releases. Things can go wrong and you can be left with a device that is flaky. So, installing the beta is at your own risk. The idea is that Apple gets you to test the software in more situations and with more combinations of hardware and software than they can possibly do in a lab.
The macOS Mojave update isn't out yet but we can expect that pretty soon.
Before installing the beta, back up your device - I recommend using iTunes to do that rather than iCloud as I find the restoration process easier and faster.
I've been running the developer beta on my iPhone and it's been surprisingly stable. The only app I've had trouble with is Evernote. It's more sluggish than usual.
If you're trying there beta out - let us know what you've found in the comments. Are there any apps that break? Is there a new feature you really like?