To do this to an entire buffer, type M-< M-x replace-regexp <RET> ^ <RET> your text <RET>.
To do this to a region, use
Set the mark (C-<SPC>) at the beginning of the first line you
want to prefix, move the cursor to last line to be prefixed, and type
M-x string-insert-rectangle <RET>. To do this for the whole
buffer, type C-x h M-x string-insert-rectangle <RET>.
If you are trying to prefix a yanked mail message with ‘>’, you
might want to set the variable
mail-yank-prefix. In Message
buffers, you can even use M-; to cite yanked messages (M-;
runs the function
comment-region, it is a general-purpose
mechanism to comment regions) (see Changing the included text prefix).