once u have done su and become the root, type
kedit /etc/name of file
This would open up the file in the kedit (only if u have Kedit installed. normally it would be installed if u install KDE.
If u dont have Kedit , u could use some other text editor
or if u have pine installed use
pico /etc/ name of file
or any other text editor, vi, vim , emacs etc.
though i thnink you should go for , kedit , or nedit for now.
emacs /etc/name of file