Some resources if you want to learn and player riichi, the japenese variant
TLDR: Download both Mahjong Soul and Kemono and play a lot. If you don't understand something then look up on the riichi wiki or ask the mahjong reddit or the Mahjong Soul Discord.
It's much much easier to learn riichi through a game then buying a set and sit down playing. Especially if it's 4 brand new players together.
Mind you riichi/mahjong doesn't have rules like chess which are set in stone. There are local rules, competitions differ, video games play differently. The core rules are the same but there are many many optional rules. A lot of confusion comes from this.
This is for example the comparison chart of popular riichi rulesets https://riichi.wiki/Comparison_of_popular_rulesets
That being said the riichi wiki is pretty good and has info about basically everything https://riichi.wiki/Main_Page
https://www.reddit.com/r/Mahjong/ is incredibly helpful for every ruleset not just riichi if you have questions like why I'm not winning (99 out of 100 times: no yaku) or which set to buy etc
https://www.youtube.com/watch?v=hlnC2rgIPrc This video if you want to play IRL. How to setup the table, the tiles, when to draw from where. Video games has the problem of "hiding" certain elements of the gameplay like how to draw from the dead wall after a kan and replenishing it.
This yaku, scoring, and teaching sheet for IRL play https://drive.google.com/drive/folders/18hxO5DMVAqxSNV9VvpjA...
If you play IRL then the Riichi compass that you can use both as a quick calculator and as the name implies a compass: you put down in the middle on a phone/tablet, set up a new game and after that you use it as an automatic score calculator https://riichi.onecomp.one/
https://discord.com/invite/mahjongsoul if you have instant questions and need help
The riichi book is the next step once you understand the game and wants to learn strategies https://dainachiba.github.io/RiichiBooks/
As for where to play digitally:
- https://mahjongsoul.game.yo-star.com/ Mahjong Soul which is a gacha game but that doesn't affect the gameplay, pure cosmetic. It has a decent tutorial, very good QoL features helping the gameplay, and a big playerbase for PvP for every rank and custom modes. You can also play 3 player riichi (sanma) and there are custom lobbies where you can play with friends and setup your own rules (I actually learned here with people on a Discord call)
- https://www.mahjong-jp.com/ Riichi City which is the newer and main competitior of Mahjong Soul. I haven't played that much here but some people prefer this but there aren't that many differences either.
- https://tenhou.net/ (https://riichi.wiki/Tenhou.net) which is popular in Japan. It has less players than Mahjong Soul but some people say at higher ranks there are better gameplay, more skillful players. ymmv if you watch japanese streamers or pros they usually both play Tenhou and Mahjong Soul and reaching the top is a lot of time so doesn't really affect new players.
- https://cyberdog.ca/kemono-mahjong/ Kemono is not the best game but it has 2 very big pros: arguably the best tutorial how to play. And it has offline mode against AI characters. So if you mostly play on phone and want to play riichi while hiking in the Himalayas then Kemono is probably the best choice. The non-traditonal portrait mode on phone is different compared to other clients
- https://www.amatsukimahjong.com/en/ Has to mention this because it’s a good one where you can play multiple rulesets easily. HK, Taiwan, Riichi