HOW CAN I FIND MY SONGS ON MY COMPUTER AND TRANSFER THE SONGS THAT I DIDNT BUY FROM ITUNES TO MY SAMSUNG S4???????

How? I don't understand why I am having so much difficulty not only trying to find the songs that I purchased through itunes but the music I already had that is only stored on my iphone at the moment. I had to re-install my computer so I lost everything that was on it. WHY??? Is this so difficult to figure out??? My iphone plays the songs, I want them on my samsung s4. How do i do it?

Thanks in advance.