If you would like to keep track of your forum topic or one where you posted a reply, add it to your Watched Topics List.
On New Topic and Full Reply screens, there is a checkbox labeled "Add this topic to my Watched Topics" below the message input box. Check that box to subscribe to updates in that topic.
After that, you can click on "My Stuff" on the horizontal menu near the top of any forum page and choose "Watch Lists." On that page, click on "Watched Topics" and then click on "Edit Watched Topics" to opt in to getting an email when a new reply is added to that topic thread.