You will first need to enroll into Apple's beta programme by following this link. But as a beta, the software will be a little buggy.
Take note that this is still beta so it won't be as stable as the official release. After, from your iOS device, visit beta.apple.com/profile, and download the iOS Beta Configuration Profile. In terms of performance, Apple said that iOS 12 will be doubling down with apps launching twice as fast, and throttling CPU to the max when needed. iOS 12 will also load apps up to 40% faster on older devices like iPhone 6 Plus. Once that's done, just go to Settings General Software Update, and you should be able to start downloading iOS 12 Public Beta.
If you want to try the beta of tvOS 12, Apple has posted a guide here.
Perhaps the coolest thing about iOS 12 is that it'll be compatible with every device that supports iOS 11.
If you have a spare iPhone, iPad, or iPod touch, then you can easily download the public beta and try it now for free. The final iOS 12 comes out this September, so we're going to have plenty of beta testing until then. There's a new Screen Time feature to see and control how much time you spend using each app. Notifications are now grouped and you can silence them from the lock screen.
When is the real version coming out?: If you're not so into trying things before they're fully baked, Apple will be coming out with a full version of iOS 12 sometime in the fall.
Here's a quick rundown of what's new in iOS 12. You also can turn on Do Not Disturb when you're in a meeting, for a few hours or for longer. Those shortcuts can be used in any app, finally allowing a whole range of apps to interact with Siri.