if i sync my ipod to my computer, will it delete everything off my ipod and go over to my itunes library?

I am trying to get my 900+ songs from my ancient ipod onto my iTunes library on my computer. Im uneasy to sync as it says everything will be removed from my ipod. will all my songs go to the iTunes library or will they be lost forever. I have nothing in my iTunes library at the moment. please help!