I would just start with the documentation available from Larian. It's important to know that there are really two different scripting languages used. There is Osiris for story scripting and then Behavioral Scripting for behaviors attached to items and creatures. While they look similar in many ways, there are some differences.
Story Scripting:
Osiris (wiki)Youtube tutorials (from DOS1 but relevant, just realize dialogues are done very differently now)
Behavioral Scripts:
Scripting