What kind of iPod do you have? If it's anything other than a Touch then how did you get the music on there is the first place? In any event, there are numerous software available that will allow you to copy over the music that's on your iPod on to you computer. DiskAid is one that comes to mind.
Download iTunes from Apple's website. You can put all of your music in there. When you want to resync it, you should be able to selectively move the songs you want on your iPod so it does not go over its storage limit.