The first class you should learn about is HTMLProject, because your add-on will ba a class that inherit from this one. You must also define a global variable, and a global function (very simple, don't worry) described here. Then you should learn about HappyLinks, the elements of the tree you'll have to build. To learn how to parse HTML files, read about the HTMLFile HTMLBeacon and HTMLLabel classes.
Have also a look at the sample add-on.