So, a couple of few things to do if you which to be notified of new replies to a topic.
Firstly, go to your User Control Panel and click on the Board Preferences tab at the top...

...then make sure the "Send subscription notifications via email" is set to "Yes".
Next, if you want to automatically set it so if you've posted in a topic you will get a email notification then click on the "Edit posting defaults" on the left...

...and make sure the "Notify me upon replies by default" is set to "Yes".
This will save you having to click the option each time you post as it will already be previously selected.

Also, if you just wish to subscribe to a topic without posting, you can find that option at the bottom of the screen:

Any questions/problems then please ask.

