You just need to sign up to become a public beta tester through Apple's website, which is free and easy.
There are some caveats you should know about, though. While you'll be getting a first-look at Apple's latest and greatest software, it also tends to be a bit buggy in the earlier stages as Apple continues to update it before this fall.
Feeling brave? Here's how to get started. Oh, and keep in mind the instructions are the same whether you're installing on an iPad or an iPhone.