If you want to take advantage of the AirPlay 2 goodness, you’ll need a device that’ll run iOS 11.4, tvOS 11.4, or macOS High Sierra 10.13.6.
Any iPhone 5s or newer will do, as will any Apple TV running tvOS (4th gen or 4K), and the iPod touch 6th-gen.
Using a Mac? Check out Apple’s macOS specs here – and you’ll need to play your audio through iTunes. Rocking an iPad? Any iPad Pro will run iOS 11.4, as will the iPad Air, iPad Air 2, iPad (2018), and any iPad mini apart from the original one.
As for receivers, Apple’s HomePod is the obvious candidate. But Apple helpfully lists third-party speakers where the manufacturers have announced compatibility, which includes the likes of Sonos, Bang & Olufsen, Bowers & Wilkins, Naim and Libratone. Need a ‘budget’ option? The best you’re going to get for now is the Sonos One.