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Simple, open google plus > menu > settings > auto backup
If you want to back up your pictures in Full resolution then use Box, you should get 50GB free
here are other alternatives
Alternatives on How to backup Xperias
That's only for pictures and videos
To transfer your iTunes music library to Google Music
Connect your phone to a computer and transfer the music directly to either your Internal Storage or SD card
You also have an option to download the music directly to your phone, just open google music, but I don't recommend this option, just transfer the music through a computer