European Mahjong Association. Here you can find all upcoming tournaments as well as ranking of all active players in Europe.
Mahjong Soul is a good place to start playing Riichi online.
Mahjong Time is a site with a selection of Mahjong variations such as Riichi, Competition Rules (MCR), Hong Kong and Taiwanese. There you can also find many tournaments.
Some professionals play on Tenhou. If you manage to pass the registration in Japanese its a nice site to play Riichi.
Efficient discarding is an important part of game. You can practice here : Trainer