(require 'package) ;; You might already have this line
(add-to-list 'package-archives
'("melpa" . "https://melpa.org/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
use
M-x package-list-packages
to list the packages.