While developers have been able to test out Apple's upcoming software updates for a few weeks now, the general public is now getting their chance. The first public betas of iOS 12, macOS 10.14 Mojave, and tvOS 12 are now available for testing.
You'll need a supported device, which in the cases of iOS and tvOS, includes the exactly same devices as iOS and tvOS 11. The means that iOS 12 will work on an iPhone 5s or better, and tvOS 12 works on a 4th-gen Apple TV and the Apple TV 4K.
macOS 10.14 Mojave is a different story. Where the previous version supported Macs going back to late 2009, this version will only support devices going back to 2012. This is a pretty big shift, so if you have an old Mac, you might be left out in the cold.
Apple still doesn't offer a public beta for watchOS, and this is mainly because there's no easy way to roll back if you find that the update is too unstable. If you're not the beta testing type, you can always wait until they're fully available this September.