Are you new to itunes?
Plug in your ipod and it will recognise that there are songs on there that are not in your itunes library and give you the option of putting them back on. Either that, or recover them from your deleted items.
When your ipod is plugged in you there is a tick option for manual synchronisation or automatic synchronisation.
You know that you just tick or untick the music you actually want transferred on to your ipod though, right? You don't need to touch the delete key.