The iOS 9 Public Beta will be available some time in July. You can sign up for it now here. The iOS 9 beta 1 is available immediately for developers, who can download it from Apple's developer site.
As part of Apple's Beta Software Program you will be able to test iOS 9 (and other products) before they are officially released to the public, and offer feedback to Apple about any issues you find. It is free to join, whereas the developer version requires you to be a registered developer, which costs $99 a year.
iOS 9 is compatible with the iPhone 4s and later and the iPad 2 2011 and later.tags: