http://mahjong-europe.org/filer/riichisheet.pdf - Riichi Mahjong Cheat Sheet; print it out, have it on hand when you're playing
http://www.tenhou.net - play online here
http://arcturus.su/tenhou/ - guide to tenhou and how to set it up
http://www.osamuko.com/ - mahjong strategy blog
http://www.reachmahjong.com/ - articles on strategy, rules, how to play; chapter 6 and 7 were particularly helpful