Learn How to Send Bug Reports
If this is your first time trying early software, you'll want to make sure you understand how to send Apple feedback about the iOS 12 beta.
If you plan on using the iOS 12 developer beta, you'll want to send feedback via Apple's Bug Reporter service. It's fairly straightforward, but you'll want to give it a look and get comfortable with it.
If you plan on using the public iOS 12 beta, you'll need to use the Feedback app on the home screen of your device.
For more on the Feedback app and submitting iOS 12 bugs and issues to Apple, check out Apple's FAQ.