You might be able to redownload some/all of your iTunes purchased music via the Purchased link under Quick Links on the right-hand side of the iTunes store homepage, or go into iTunes > Preferences and on the Store tab tick the 'show iTunes in the cloud purchases' tickbox, what you can redownload will then show in the My Music part of your library with a cloud icon against it for redownloading.
For your own music (e.g. copied from CDs), and music that you can't redownload (e.g. if it's no longer in the store or you've moved countries since buying it), you will either need to add it form your backup of your downloads/library (File > Add To Library in iTunes) or copy it over form your old computer : How to move your iTunes library to a new computer - Apple Support
or iTunes for Windows: How to copy purchases between computers - Apple Support