The developer beta of iOS 9 has been available since WWDC in June. However to get it you needed to sign up for one of Apple's developer accounts. Yesterday the beta was opened up to anyone of the general public that has an Apple ID and the time to install it.
Note: With iOS 9 there will certainly be teething problems in terms of app compatibility, troubleshooting issues and crashing, remember it's a beta.
To get the public beta for iOS 9, simply follow these steps:
1. Back up your iOS device.
2. Sign up for the Apple Beta Software Program with your Apple ID and password.
3. From your iOS device go to beta.apple.com/profile and follow the onscreen instructions to download the profile.
4. Restart your device.
5. After your device restarts, launch the Settings app and tap on General followed by Software Update. It may already be downloading.
6. Once downloaded you're ready to go. Remember to report any issues to Apple, they need to iron out as many problems as possible before the release.