Before it can be installed on your device, you must sign in with your Apple ID at beta.apple.com in order to enroll your iOS device to access the latest public beta of iOS 11.
Follow the onscreen instructions to allow installation of the profile, which will permit your iOS device to automatically receive over-the-air public beta software updates.
Lastly, go to Settings → General → Software Update to install the public beta.
iOS betas are typically riddled with bugs and various issues until a Gold Master candidate is released ahead of public launch. You’ve been warned—performance issues, crashes, battery drain and various other problems and hiccups are a commonplace with iOS betas.