(require 'package) ;; You might already have this line
(add-to-list 'package-archives
             '("melpa" . "http://melpa.org/packages/"))
(add-to-list 'package-archives
             '("melpa" . "http://melpa.milkbox.net/packages/"))
(when (< emacs-major-version 24)
  ;; For important compatibility libraries like cl-lib
  (add-to-list 'package-archives '("gnu" . "http://elpa.gnu.org/packages/")))
(package-initialize) ;; You might already have this line

(package-install auto-complete)
(package-install ac-math)
(package-install ac-c-headers)
(package-install ac-octave)
(package-install auto-complete-clang-async)