How do I 'unshare' an Apple ID with a family member?

My wife and I share one Apple account. We both now have Apple Watches, iPhones, and iPads. We also share one laptop. We are having difficulty keeping data straight on each. I understand that we should both have separate Apple IDs; one of you cautioned us to clean up data on them before making the change. We are not sure what that entails. Can anyone provide or “point to” details on how to do that?

[Re-Titled by Moderator]