Hi
First of all you need to run vi with an argument of file you want to edit, eg:
vi /etc/X11/XF86Config
then it would be wise to write
':help' and press [enter] to get help ;-)
also you may want to take a look at 'man vi' directly from the console ;-)
but for this moment when you run vi you just need to press 'a' to be able to enter text, and after your corrections press 'escape' to leave write mode, then enter ':w' and press [enter] to save and ':q' and [enter] to quit
those ':' are important, but '' are just quotation marks
Take care,
Alex