The Bay of Plenty referee society has a page with an online course on introduction to refereeing rugby. There is a downloadable workbook and a series of videos. The modules are:
Introduction and Welcome
Module 1: The Role of the Referee
Module 2: Fixture Organisation
Module 3: Whistle! Signals! Verbals!
Module 4: Where do I stand?
Module 5: The Kicks of Rugby
Module 6: Tackle Ball carrier brought to ground
Module 7: Ruck and Maul
Module 8: Scrum
Module 9: Lineout
Module 10: Offside and onside in general play
Module 11: Foul Play
Module 12: Advantage, free kicks and penalty kicks
Module 13: Assistant Referee/Touch Judging
Module 14: Method of scoring – ingot
I don't love their list of reasons for becoming a referee, listed below. I'm pretty sure it's not a comprehensive list. But the course looks pretty good for a beginning ref.