The button is found at the very bottom of a thread's page, second from left: A checked box icon with the text "Subscribe Topic".
Click it, and a confirmation message will be displayed before returning you to the thread.
To manage your subscribed threads and forums:
* open the User Control Panel (the link for it is at the top left under the board breadcrumb links in the light blue bar), then
* open the Options tabsheet, then
* open the Manage Subscriptions tabsheet.