As James said it really depends on what type of backup you want to do.
The way I'd go about this is to only back up the home directory. This has all the users files and most of the look and feel configuration.
I'd also backup to external (USB) HDD although a pen drive might be adequate depending on how much stuff you have on the machine. first mount the drive in the way you do normally. (should be automatically mounted in linpus)
Then you need to decide whether you want to do a clean backup every time, which takes a lot of disk space and time but allows you to cycle though old documents and is slightly safer or a backup which updates files each (quick and easy and pretty safe).
For the former take create a directory called acerone_bak_05062009 (you can use any name but I like this one the numbers are the date and tells you what it is) either using you file manager or the mkdir command and then simply copy the important files (in this case /home and everything below it) again using the file manager or
Code:
#cp -r /home /media/HDD/acerone_bak_05062009
For the latter again create a directory to back up into but you don't need date information and run
Code:
#rsync -avh /home /media/HDD/acerone_bak
This will make a copy of new files and overwrite edited files
You need to change the paths to be appropriate and to backup the whole of home you need to run as root (su/sudo) for all the commands
You could compress the directory (into a tar.gz, tar.bz or .zip file) then but if you use rsync I think you need to uncompress the files first.
