I suppose the first thing to look at is the laws. World Rugby posts the current laws along with a lot of ancillary information on their site. https://laws.worldrugby.org/
There's also the Playing Charter and Foreword (really, read them), application guidelines, law clarifications, referee signals, variations, and something called XRugby that I have no idea what it is. You can also get the laws (but not most of the other stuff) in an iPhone app. They might even still print the laws in a little book, but if they don't you can print it yourself from the pdf below.