Go to your user CP, the link is in the top left corner. This will take you to a page with a lot of links on the left hand side of the page. Look under settings and options and you will find a link that says edit options. Click that link and then look on the page that takes you to for where it says default thread subscription mode there is a drop down menu which you have to change to no email notification
Thats the first step.
second step is to click the user CP link again and to unsubscribe from all the threads you have already posted in because these threads will already be sending emails when someone posts in them.