how do I transfer my library from my PC to my new iMac

I bought a new iMac. I have an extensive library on my old PC. When I transferred my iPod Touch over to my iMac, everything was good except the pieces of music and things I had bought or downloaded other than from the iTunes store were not there. I found how to click the little arrow next to the iPod Touch window when my iPod Touch is in the dock for the iMac. It will then show me all the things I have loaded on my iPod Touch. But when I try to play them or transfer them to my computer (iMac), it has a little ! by each song or recording. I don't have access to them. I also can't figure out how to get to them on the actual iPod Touch. I think I have lost them all. Help!! Thanks.