Go the official forum and you will find all the answers :P
Go here for quests and items info.
http://www.portcoimbra.com/
You get stances from quests or buy them from Masters in the Masters room (each city has it). When you buy them you can learn them buy using the book from your inventory, but you first need to meet all the requerments. After you learn it, you can change the stance in the combat area by clicking the appropriate button or by equiping the weapon that the stance is for and it will automaticly change.