https://www.theverge.com/2018/6/26/17501302/macos-mojave-public-beta-download-now-available-how-to-install

Apple has opened up a public beta for macOS Mojave, the next version of its desktop OS. Anyone with a supported Mac (and that’s most of them made from 2012 onward) can install and start to try out the new OS today. Its new features include a dark mode, a redesigned app store, four new ports of iOS apps, a new option for cleaning up your desktop, and a new view in Finder, among a many others.

Be warned that, since this is a beta, not everything is working yet and you’re definitely going to come across bugs and an unusual number of crashes in some places (especially those iOS apps). Most things are working well enough, but be careful if you’re thinking about installing this on your main computer, since there’s no guarantee that you won’t run into issues with a critical app you need for work or whatever else.