A subscription to a thread can mean a number of things depending on what options you select. It will either be listed on your control panel screen when it's posted to or you will receive email notifications when it's posted to. If you want to reply to an existing thread just go to that thread and on the left side just above the first post and below the last post there's links that say "post reply" that will take you to a form to enter your post. Or you can click on the quick reply button at the bottom of any individual post and that will take you to the quick reply window and you can post there. If you want to start a new thread just go to the forum you want to post in and click on the "new thread" icon on the left at the top or bottom of the list of threads.