The developer beta of OS X El Capitan 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.
To get the public beta for OS X El Capitan, simply follow these steps:
1. Sign up for the Apple Beta Software Program with your Apple ID and password.
3. On the screen that comes up, go to step 2 and click Download OS X El Capitan Public Beta.
4. Enter your Apple ID and password in the App Store. Your redemption code should already be in the box, if it's not, enter it.
5. Your beta should automatically start downloading.
If you want to see what's new in El Capitan, go here.