Open OneCare, select Change Settings, and go to the Backup tab. Follow the wizard to specify what you wish to backup, where to backup, and when to backup. Be sure to review the file types and what will be backed up as you may be backing up much more data that you wish. You can exclude folders or files from being scanned for files to backup, to reduce the amount of data being backed up.
You can also choose to not backup anything, by unchecking "turn on backup on this PC" which will put you back to green status, but in the event of a problem that requires a drive replacement or reinstall of the factory image, you will have no backup to use to recover any of your data unless you are using a different backup solution.
-steve