If you want to subscribe to a Forum to receive an email notification whenever there is a new topic, you can just click the Subscribe for new topics link after you click on a Forum.
You can also subscribe to an individual Topic (which is basically a discussion thread).
There are two options for subscribing to an individual Topic:
1. While you are replying, click the Subscribe to this topic link:
2. While you are viewing a Topic, click the Subscribe for new replies link at the top of the thread: