Let me see if I understand you completely.
You bought a used iPod...
Did it come with stuff (apps, music, etc) already on it and you need to remove it and add your own from iTunes on your computer?
HT4137 Will help you restore the device as new, erasing all the information on it, as well as making sure the iOS software is up to date. Then you can sync with iTunes and put whatever you want onto the iPod.