To add a code block, surround it with #+BEGIN_SRC language and #+END_SRC. language should correspond to the major mode for the language in question, e.g. the major mode for Emacs Lisp is emacs-lisp-mode, so write #+BEGIN_SRC emacs-lisp.
#+BEGIN_SRC emacs-lisp
(defun hello-world ()
(interactive)...