If you are new to modding it is recommended to first explore the 'Getting started' section. It will allow you to follow a few tutorials to give you a basic insight of how modding with XStoryPlayer can be done.
For more advanced modders the section 'Advanced modding' will give you hands on advice on how to do special things like adding animation, making a characters brain, NLP etc.
Finally there is also an overview of the script language. In it all current supported functions are described.
For making the tutorials you need to have the XStoryPlayer Beta Patch installed. (Re)download and install it before starting the tutorials, even if you have it installed already.