When you make a post to a phpBB forum, there is a checkmark option where you have to checkmark in order to have phpBB notify you whenever someone replies to your post. See tutorial on first phpBB post. However you can set your user preference so that this checkmark is checked by default.

1. First login as that user. For the purpose of this example, we log in as admin. And you will see a "User Control Panel" link. Click that.

2. Click the "Board Preferences" tab and click "Edit posting defaults"....

Set the option "Notify me upon replies by default" to "Yes". Click the Submit button.

Note: this tutorial was written based on version 3.0.5 of phpBB