Chifmunk, if you created the forum thread, you will automatically be already subscribed to it.
If you want to be subscribe/follow a thread that someone else created, just click the 'target'/Follow button to turn it yellow (active). See my attached screenshot below to see the button in this thread, for example. When the Follow button is yellow, you are actively following a thread.
To adjust what kinds of actions or items you want to be Notified about, go to your own profile page (click on your name link anywhere) and go to the Notifications TAB to select your preferences about getting site notifications.
Those irritated by grain of sand best avoid beach.
-Strumelia proverb c.1990