I know that \"C-u 80 C-x f\" sets the fill width 80, and M-q adjusts it. How to make it default? I mean, how to make width 80 deafault for fill width?
In your .emacs place (setq-default fill-column 80) Or you can just use M-x customize-variable on fill-column, and it will set it and save it for you using the standard customize interface.
(setq-default fill-column 80)