Mac showing storage full and don't know why

I have just bought a mac and there isn't enough storage to put all of my old files on. I have moved a folder over containing all of my music which was fine, when I tried to import this into iTunes it said I had filled my hard drive so I cancelled it and deleted all songs from iTunes and my other music folder. My laptop still shows 50gig of music on the hard drive though. How can I clean this up?