Okay I guess you haven't spent much time on CLI

You need to be root to use those commands (the # was the hint; a $ is a hint to run as normal user)

Add a sudo before those commands (assuming of course whichever method of obtaining root allows for it)

$ sudo apt-get -f install
$ sudo apt-get update
$ sudo cat /etc/apt/sources.list

you might instead use becomeroot or su - or root (easyroot) to run a command a root depending on your situation.

There are a lot of ways to backup and they each have their pros and cons. Of course you might be in a catch-22 situation to be able to use something like http://talk.maemo.org/showthread.php?t=58633
So I suppose you could connect your device to a desktop and copy over any files that are important to you (such as the contents of /home/user/MyDocs )

Flash instruction vary whether you have a N8x0 or N900 but see here:
http://wiki.maemo.org/Updating_the_tablet_firmware