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You will need to pay $99 and join Apple’s iOS Developer program. However, are you really sure that you want the beta on your iPhone? It’s beta for a reason - it’s not ready for public use yet. There are numerous known issues with iOS 8 beta, and they include potential poor battery life, improperly functioning iCloud backup/restoration, disappearing preferences in apps, sync failures for contacts, game center crashes, 3rd party keyboards won’t work properly, failure to wake from lock screen…I could go on, but you probably get the idea. Instead of a stable, well polished user experience, you are going to get an OS that is somewhat unstable, has broken features and isn’t fully compatible with applications and accessories. I understand the temptation, but seriously suggest that you wait unless you are a developer or at least really know what you are doing.
If you really want to do it though, here is a link to a complete guide on how to do it. Good luck.