syncing my iPod with my iTunes

I have had my iPod since June and my PowewBook for 2 years. I just had to have Apple replace the hard drive in it. Since I lost everything on the computer I want to "download" the music I used to have in iTunes (which is currently on my iPod) back into iTunes, but can't figure out how. It seems I can easliy upload music from the computer to the iPod, but I'm not seeing how to go the opposite way.