If you are typing a comment and wish to continue it on another line,
you can use the command M-LFD
(indent-new-comment-line). This terminates the comment you are
typing, creates a new blank line afterward, and begins a new comment
indented under the old one. When Auto Fill mode is on, going past the
fill column while typing a comment causes the comment to be continued in
just this fashion. If point is not at the end of the line when
M-LFD is typed, the text on the rest of the line becomes
part of the new comment line.
To turn existing lines into comment lines, use the M-x comment-region command. It adds comment delimiters to the lines that start in the region, thus commenting them out. With a negative argument, it does the opposite--it deletes comment delimiters from the lines in the region.
With a positive argument, comment-region duplicates the last
character of the comment start sequence it adds; the argument specifies
how many copies of the character to insert. Thus, in Lisp mode,
C-u 2 M-x comment-region adds `;;' to each line. Duplicating
the comment delimiter is a way of calling attention to the comment. It
can also affect how the comment is indented. In Lisp, for proper
indentation, you should use an argument of two, if between defuns, and
three, if within a defun.