Ipod shuffle music

I work on a mac, I just got a ipod shuffle…because I didn’t want to spend tons of money. The first songs I added were from my friends pc, it didn’t do any of syncing, but I put about 150 songs…then I came to work to charge it, plugged into my mac, my itunes opened, and someone had some songs on there before me. My songs did not show up, and all those songs got added, I went back in and deleted those, and then made a playlist, because in my finder window I could see my ipod with my songs as files…so I copied my songs, made a playlist and added the play list through itunes, and then it synced and connected, etc…now my songs are on there. My question being, do I have to burn music files from friends computers, or put files on a flashdrive and then add a playlist to my itunes (well my works itunes) and then add, and it won’t delete what I already have..? I am confused, but I am glad I got some on there, and glad I didn’t spend a lot of money, they should really tell you this info when you buy it or before… So anyone know if I am crazy or if that is the way I will have to do it..I can’t just plug my ipod into a friends and get their music…but can I save to disk/flash drive and then add??? Help>>> I am very computer literate, I work on both mac and pc and this is just ridiculous…!! Thanks for any info you have