(require 'package)
(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
;; Comment/uncomment this line to enable MELPA Stable if desired. See `package-archive-priorities`
;; and `package-pinned-packages`. Most users will not need or want to do this.
;;(add-to-list 'package-archives '("melpa-stable" . "https://stable.melpa.org/packages/") t)
(package-initialize)
Note that you'll need to run M-x package-refresh-contents or M-x package-list-packages to ensure that Emacs has fetched the MELPA package list before you can install packages with M-x package-install or similar.