it is different on all computers but there is a button on the bottom right hand corner (all computers) that when you put your mouse over it. It says safely remove hard ware or something like that and...
You can either click the eject button by the iPod menu in iTunes, manually eject it from the computer by doing what your computer normally does to eject a flash drive, or you can just unplug it.
Make sure that the USB cord is plugged in at both ends. If that doesn't work, try a different USB from someone else's iPod. If that doesn't work, make sure that iTunes is installed on the computer....