You can subscribe to a forum and all the boards within the forum, or you can choose which boards you wish to subscribe to. Subscribing will get you an email notification every time a post is made unless you change your email preferences.
Click on the forum and you will be taken to the forum home page
On the landing page, click the green subscribe button
When you click subscribe you will get a pop up that will allow you to choose the forum and all the boards associated or individually choose which boards you may want to subscribe to.
You can also choose to subscribe to boards individually. Navigate to that board and on the right side of the page, click the green button that reads subscribe (same as example 2 above).