Depends on which editors you have installed? Try nano, pico or mcedit as those are fairly simple and you can see commands at the bottom of the screen.
If you don't have those try vi, or rather vim. For that one you might need to take a look at: http://www.tuxfiles.org/linuxhelp/vimcheat.html
EDIT: oops, forgot: The usage is for example: nano /etc/X11/XF86Config-4