I don't believe there is anything you need to do. When you post to a thread, the envelope at the left margin will then have an arrow on it noting that you posted to the thread. If you want to subscribe to the thread (whether you've posted or not), you can use the "thread tools" button to do that.
if ya click the triangle by thread tools, above where the posts are, one of the options is follow, or track, or something like that, from this screen I can't see it.