If you downloaded music from the apple store then yes you would need to pay for it. However you can also download iTunes and use it to rip music from CD's to your iPod or import files from your computer to your iPod. Neither of those options will cost a penny.
Download iTunes. There will be a menu that says add music or import. Click on it. Locate music that is stored on your computer. It will ‘copy’ it to iTunes. Connect iPod. Sync. itunes comes with a ‘how to’