That depends:
-- on the storage capacity of your Nano
-- how many songs you want to transfer
-- do you have all of these songs on your computer already or are you trying to combine the song catalogs of two different ipods into your computer and then move them to your new Nano?
--are you on a Mac or PC?
As you may know, the songs on any given ipod are linked to the itunes library of a specific computer. Apple (and the record companies) do not want you to be able to take an ipod full of music over to your friend's house, for example, and give them a copy of all the songs on your ipod. If you connect an ipod to your computer that is not already "linked" to the itunes library on that computer, your computer will ask you if you want to replace the contents of the ipod with the song library on the computer.
It IS possible to get around this limitation, but doing so is not for a computer novice.