You can update or change various options, email digest, and notification settings after you sign-in by selecting the Settings link when you click on your avatar at the top right of any Communities page.
If you are not receiving an email alert when another member replies to your forum post, check the "Notifications" tab in the Settings area, then check the box in the "Email" column next to the items in the first section, "Activity from subscriptions". Scroll to the bottom, then click "save".
If you do not wish to receive an email alert for every post within a particular Forum, unsubscribe (turn notifications off) - to do this, navigate to the Forum in question, and click the button on the right side that says "Turn Forum notifications off". It should then read "Turn Notifications on", indicating that they are now off.
If you only want to receive email alerts for forum threads that you start, or reply to, this happens automatically, as long as you have the box checked in your profile settings as explained above. In other words, when you start your own thread, you will be automatically subscribed to that specific thread. When viewing a particular thread, if you click the "more" button below it (below the original post) you should see "Turn reply notifications off" (indicating that they are currently turned on - to send you email alerts, if the box is checked in your profile Settings).