To get the beta, you must sign up for the Apple Beta Software Program, which requires an Apple ID. The Sierra beta installer is downloaded through the Mac App Store.
Before running the installer, be sure to back up your data. Remember, this is beta software, and it’s possible that some of the software you have may have problems.
There are a couple of conditions that Apple has if you want to use the Sierra beta:
Installing the beta will not void your Mac’s warranty. And when the final version of Sierra is released this fall, you can upgrade from the beta.
If you install the beta and decide later that you want to go back to your previous operating system, you have erase your drive completely, reinstall the previous operating system, and then restore your data from a backup.