HTML5 is a markup language of the internet used for structuring and presenting content for the WWW and bla bla. You can read all the detailed explanations for what HTML5 is from Wikipedia. This tutorial is not for that as I am trying to be more practical and provide you with fast start.
First you need an editor. You can work with notepad or something more fancy. I prefer to use something with code completion and some other useful features. Good choice for this is Netbeans. There are some lightweight editors but I haven't tried any of them as Netbeans is doing good work for me.