VIM: Insert a string at the beginning of each line

This replaces the beginning of each line with "//":

:%s!^!//!

This replaces the beginning of each selected line (use visual mode to select) with "//":

:'<,'>s!^!//!

Refer:

http://stackoverflow.com/questions/253380/how-do-i-insert-text-at-beginning-of-a-multi-line-selection-in-vi-vim