Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
While you can use the barest of barebones text editors to create HTML and XHTML documents, most authors have a toolbox of software utilities that is a bit more elaborate than a simple text editor. At the barest minimum, you also need a browser, so you can test and refine your work. Beyond the essentials are some specialized software tools for developing and preparing HTML documents and accessory multimedia files.
At the very least, you'll need a text editor, a browser to check your work, and, ideally, a connection to the Internet.