>>327310302
It's recommended you start out with Mahjong Souls, f2p on browser and phone. It's very good at handholding and is very easy to learn, just takes a little while to get the grasps.

There's also videos in this post >>327216503 for getting the basics. And if you don't mind skimming thru a pdf there's a reiichi book https://repo.riichi.moe/books/rb1.html for learning tile efficiency which is IMO 100x more important than learning yaku, you can skip the entire 1st part about reiichi online and you don't HAVE to read all of the book to get more than a beginneres/intermediate grasp on Mahjong. But reading/skimming aside just play some games on Mahjong Souls is more than enough to help you be efficent