This iMore article walks you though the steps necessary to create an Apple ID which is required to download content from iTunes and the App Store. It also explains how to use your device to log into, or log out of, your iTunes account.

Your Apple ID allows you to sync reminders, calendars, and contacts through iCloud, and activates FaceTime and iMessage across multiple devices. 

It's not mandatory to sign up for an Apple ID to use an Apple device, but it is essential if you want to get the most out of your iPhone and iPad. 

Click here to see a support article "How to change your Apple ID to a different email addres".

Click here to read another iMore article "How to switch Apple IDs on your iPhone or iPad"

This support page tells you how to use iTunes on a Mac or PC, you can view a list of devices associated with your Apple ID and remove the devices that you no longer want associated.

If you search for "how to transfer itunes for one user to another" you will get a lot of help for sharing iTunes resources, with limitations.