Towards the top right hand corner of this page is an e-mail icon in the shape of an envelope. To enable automatic notification of a reply you your post, click on that. I think it's true to say that if you quote someone's post or part of it, the notification becomes automatic.
It's definitely true to say that if you write a reply then click that envelope to test what you just wrote is right, you end up having to write it all again! I know because I just had to.