It's actually a very easy thing to do, I think a visual representation of the process will explain better.

STEP 1:
At the top right or button right of your page you'll see the "notify" option there.


STEP 2:
Click on the "notify" and you'll see a pop-up message asking you to confirm it.


Note: that's if the thread has already been created.

you can also enable it during the creation of a topic.

STEP 1:
Click on the "+Additional options..."


STEP 2:
Make sure the checkbox of "notify me of new replies" is checked.


that's it you're good to go.