Go to http://www.aptana.com/products/studio3/download, and download the correct version of the Aptana Studio IDE for your operating system. Double-click on the file once it has completed downloading, and follow the installer directions to install Aptana Studio.
By no means should you feel like you have to use Aptana Studio. There are a number of other good IDEs out there, and you may already be using a different IDE or may just be happy using your favorite text editor. However, I will be using Aptana Studio throughout the book, so it may be easier to follow along if you are also using Aptana Studio.