5.33 How do I insert <some text> at the beginning of every line?

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 string-insert-rectangle. 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).