Use "M-x list-packages" go to package-menu-mode. Then use up/down arrow key traverse items, use "i" mark the current item for installation, use "d" for uninstallation, use "u" for unmark, "x" for execute installaiton/remove.

Or use "M-x package-install" to install a package.

Ref: A Guide on Emacs 24 Package System

Modify repository and take effect

Modify ~/.emacs to add repository like the followings, then run "M-x eval-buffer".

Use MELPA repository

(require 'package) 
(add-to-list 'package-archives 
   '("melpa" . "http://melpa.milkbox.net/packages/") t) 
(package-initialize)

This works in Yiyang intranet.

Use Marmalade Repository

(require 'package) 
(add-to-list 'package-archives 
   '("marmalade" . "http://marmalade-repo.org/packages/")) 
(package-initialize)

This doesn't work in Yiyang intranet.