Emacs version 24 is available via the software center from Ubuntu 13.10 onwards. If you have an older version of Ubuntu then you can use an Ubuntu personal package archive (PPA) on launchpad.net https://launchpad.net/~cassou/+archive/emacs
Open a terminal window and run the following commands to add the personal package archive to your software library and install Emacs 24
sudo apt-add-repository ppa:cassou/emacs
sudo apt-get update
sudo apt-get install emacs-snapshot emacs-goodies-el
The best method I have found is to use EmacsForMacOSX, although some developers prefer Aquamacs or using homebrew. Have a look at the article Getting Started with Emacs 24 for MacOSX.
Gnu Emacs on windows