Mahjong Copypasta

For beginners
Everything you need to know about tenhou itself can be found here http://arcturus.su/tenhou/

Explanations for the tiles, rules, scoring etc can be found here:
http://www.ofb.net/~whuang/ugcs/gp/mahjong/mahjong.html
http://arcturus.su/~alvin/mj/sywtpmj-1.4.doc
http://www.delfosse.com/mahjong.html
http://www.japanesemahjong.com/reachmahjong/intro.htm
http://www.sloperama.com/mjfaq/mjfaq01.htm
http://www.reachmahjong.com/home/index.php?option=com_content&task=view&id=69&Itemid=77
http://www.osamuko.com/2009/04/10/top-10-hands-you-should-re-memorize-if-you-ever-lose-your-memory/

http://arcturus.su/~alvin/mj/riichirules.pdf
http://arcturus.su/~alvin/mj/riichisheet-ema.pdf
http://arcturus.su/~alvin/mj/riichisheet-dk.pdf
http://www.reachmahjong.com/home/index.php?option=com_content&task=view&id=99

Games to test your Knowledge:
http://www.gamedesign.jp/flash/mahjong/mahjong_e.html
http://arcturus.su/tenhou/scorequiz/yaku.html
http://arcturus.su/tenhou/scorequiz/

Please read 2-3 sites before playing and even then try the singleplayer games first.
Playing without knowing what you do is not only frustrating for yourself but also for your opponents, a player that left because he couldn't ron for his no Yaku hand or because someone ron'd his discard just kills the whole fun for everyone.

Usually, a game lasts around 30 minutes, so please, if you're not going to stay until the end, respect the other players and don't join a new game.

Also keep in mind that Mahjong is more a game of "not losing" than straightforward winning.
Furthermore you play with 3 other people and luck plays a big part in it, so dont expect to win more than 1 out 4 games, especially not as a beginner.

If you read some of the linked sites and still have questions feel free to ask.