Apple has made the public iOS 12 Beta live and can be downloaded by anyone and not just developers. The best part about the Beta programme is that any user can enroll for free and download the public beta profile, compared to the developer beta which requires a $99 per year developer account.
In order to download, the easiest thing to do is just head to beta.apple.com on the iOS device you want to install the beta profile on. Enroll your device, double-check that you’re backed up, and hit download. As with any iOS update, you’ll want to be fully charged (or plugged in) and connected to Wi-Fi.
You can find the list of Apple device compatible for the iOS 12 Beta here.