macOS High Sierra Public Beta is available for download now without needing an developer account. Everyone that wants to try it out can, free of charge.
The final release is expected around fall 2017. If you like to play around with new software and accept bugs this is for you. I wouldn’t recommend installing this on a mission critical machine. As Beta software can be unstable at times. However if you would like to try out new features like Metal 2 and the AFPS filesystem. Go ahead!
Apple Beta Software Program
You need to enroll your computer in the Apple beta software program to get access to the beta. Go to Apple Beta Software Program and Sign Up or use your Apple ID if you have on to sign in.
Follow the instructions on that page to enroll your device.
Eventually you will be able to download the macOS High Sierra public beta access utility. Do that, and when it finishes open the file and run the installer. When this is finished the Mac App Store should open and you will be able to go to the Update screen in the App Store and macOS High Sierra Beta should be available for download.
Make sure you have an backup of important files before you begin in case things go south. Remember, this is beta software after all and things can break. Especially now when Apple is in the transition of changing to a completely new filesystem.