How do I get the beta version of iOS 18?

If you are a registered developer with Apple, you have access to the beta.

Otherwise, you have to be invited by Apple to have access to the beta. You would have known about a month ago whether you were invited.

The purpose to release to the developers is so they can test their apps on the new OS to make sure they will work. It is not released to the general public because there are features that are not complete and it should not be installed on a primary device.