Apple has released the first beta of iOS 9 to developers.
Developers can also grab the watchOS 2 beta configuration profile. This is needed for developers to install watchOS 2.0 on their devices. They will also require the iOS 9 beta, which should be installed first, as well as the iOS 9 SDK. That can be found at the watchOS Developer Center
iOS 9 will also be available as a public beta at some point in July.
iOS 9 was announced during Apple's keynote at WWDC 2015. It features a number of enhancements to user experience and performance, including a new and improved Siri, transit maps, split-screen multitasking for the iPad, and more.