Apple has opened the public beta program for iOS 10 and macOS Sierra today in order to get more feedback on bugs and features from a larger number of users. If you're not already signed up for the public beta program, you can do so here.
The public beta program is free, but that doesn't mean that most users should run out and install the new operating systems. Beta means that not all advertised features and functionality is in the most current releases, and it also means you may very well encounter nasty bugs that you'll want to report. And never install beta OS versions on your primary devices; it's recommended that you only install the beta software if you have secondary Macs or iOS devices for testing.
That being said, there are some cool new things you can look forward to with both iOS 10 and macOS Sierra. The new Lock screen, and Photos, News, Music, Home and Messages apps on iOS 10 are quite fun to work with, and Siri on a Mac is just like having your own, personal HAL 9000 computer (except it will open your pod bay doors, Dave).