From emacs 24.4 package.el is avalable, and one way to install helm is to do it via MELPA. First, add the MELPA repository as package archive by putting following code somewhere in your ~/.emacs (or, ~/.emacs.d/init.el).

(require 'package)

;; add the repository before the package-initialize.
(add-to-list 'package-archives '("melpa" . "http://melpa.milkbox.net/packages/"))

(package-initialize)

Next, enter M-x list-packages to see the avaiable package list. Search for helm entry, put your cursor on the helm entry, press RET. You'll see the package information buffer. Put your cursor on [Install], and press RET. Helm will be installed. The package list window and the package information window is shown in following image.