Q: how do I become a bata user

You become a developer.

First you need to learn to spell, "Beta"

Then you go here: https://beta.apple.com/sp/betaprogram/

You don't have to become a developer but you will have to agree to a Non-Disclosure Agreement (NDA) so you will not be able to discuss this with anyone nor show it off. It will also be buggy at the start when it is released in July so expect problems and report them. You could even end up with a non-functional watch until a newer build is released.

As this was posted in the Apple Watch section, I assumed the OP was talking about a beta tester for watchOS which is not part of the public beta program. The only way to test beta watchOS software is to register as a developer. As of today.

True, but if Apple does decide to do so they would need to also be a Public Beta tester or iOS 9 so that would be a good place to start.