I'm not very tech-savvy, so this is the way I've been doing it for my past two Samsung phones (Galaxy S2 and Galaxy S4):
I have iTunes and Kies on my laptop. I only use iTunes to import music from CDs (I've found it's faster and more efficient), then transfer the music to my phone via Kies. For purchasing music, I simply use the Google Play store app on my phone instead of purchasing through iTunes on my laptop and then transferring the music over.