Do you want to use more than one iPod with the same computer? There are a couple of different ways you can do it. Apple has posted an article about how to do so.
If you share your computer with other people, the simplest way is to maintain separate user accounts on the computer for each person and his or her iPod. This is useful if everyone has different musical tastes; each person gets to maintain his or her own personalized iTunes music library, and can customize the iPod with his or her music.
If one person wants to use multiple iPods on a computer, or you don’t want to create multiple user accounts to support multiple iPods, you can set iTunes to update each iPod with only certain playlists. This way, you can put all your music on, say, your iPod photo and keep just your workout music on your iPod mini. First, create a new iTunes playlist for each iPod that contains all the music that you want to send to that particular iPod. Then configure the iPod Options in iTunes so that each iPod gets updated with only your selected playlists.
Learn more about how to use multiple iPods with one computer here.
If you create multiple user accounts on one computer but want the same music to be available in iTunes for all users, see “iTunes: How to share music between different accounts on a single computer.”