If you wish to keep the songs you have on the iPod you must first follow this tutorial to copy the iPod's songs to your iTunes Library:
Launch iTunes by selecting its icon in the dock
Connect your iPod to the computer. Once it shows up in iTunes press the Restore button under the Summary Tab.
A popup will appear warning you that all the songs on your iPod will be erased. Click the Restore button.
An iPod Software Update window will open. Click the Next button.
Click the Agree button to accept the license agreement.
iTunes will now go and download the latest firmware for your iPhone.
Once installation begins it will ask you for an Administrator's username and password. Enter this information then click the OK button.
iTunes will then format and update your iPod after which it will reboot.
Once complete you will be asked to name your iPod. Do this then press the Done button.
iTunes will now automatically begin syncing your library to the iPod!