How to use only one Apple ID for all my devices?

For some reason, I have multiple Apple IDs and for each item that I need one. Then I try to use one or another and they're not accepted.

How do I begin to have only on ID for everything- App Store, iTunes, iCloud etc etc. that work on all my devices. That's something else I experienced. The ID for App Store, for example on my computer doesn't work on my iPhone. For example.

I'd really like to clean this all up. I'm looking at the ID in the system prefs- it's a @mac.com. But the passwords differ everywhere I go. Some are @me.com or something else.

How do I begin to fix this?

Thanks, Tzipora

[Re-Titled by Moderator]