how can you backup your device on the computer
Sync with iTunes.
Please Get the iPod Touch User Manual for iOS 5
iOS: How to back up
The backup does not included synced media like apps and music.
You can redownload iTunes purchases by:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
You can backup your iTunes library by:
iTunes: Back up your iTunes library by copying to an external hard drive
Retrieving data ...