To subscribe to a thread, at the top of the page click on Thread Tools. One of the options should be "Subscribe to this thread." Then you will have some options about receiving email notification, etc.
You can also subscribe when you make a post. Down at the bottom of the page where you type your reply (or your message, if it is the first post on the thread) there is a button for Subscribe.
You can also subscribe to a whole Forum (e.g., the Edge) by clicking on "Forum Tools" at the top.