How to put music into itunes that was put on my iphone from another computer?

about 80% of the music on my iphone 3gs was put on my phone from my buddy on his computer. I just bought a new computer yesterday and i figured out how to put the music that i purchased from itunes off my phone into itunes but the rest of the music was put on through my friends comp. pleeze help me figure out how to get the rest of this music onto itunes. Also, please put it in simple terms. Im very computer illiterate. Tnks