Backing Up iTunes 11 Library
It sounds like you have gotten a little confused with the difference between backing up your iPod and backing up your iTunes library. You can back up your iPod to iTunes on your computer, but this is different than backing up your iTunes library itself - all the music, movies, books, apps, and such that you have placed in iTunes. 

If you want to backup your iTunes library
(Apple.com):


Open iTunes Preferences.

Mac: Choose iTunes > Preferences.
Windows: Choose Edit > Preferences.

Click Advanced.
Make sure the option to "Copy files to iTunes Media folder when adding to library" is selected.
Choose File > Library > Organize Library.
Select the option to "Consolidate files."
Locate your iTunes library. 

If you can't find it, open your iTunes Preferences and click "Advanced." The location of your iTunes folder will be listed in the "iTunes Media folder location" box.

Copy the iTunes library directory over to your backup location. 




If you really would like to back up your iPod:


Plug your iPod in and view its details.
Under "Summary" tab there is a heading called "Manually Backup and Restore"
Select "Back Up Now" from under that heading.


For more information on backing up iOS devices in general, this article is a good start.