The thread title says it all 
How can i enable notifications or follow someone or some thread ?
If you reply in a topic your are automatically subscribed to it and will receive notifications in your email. Also if you click on the Circle under the scroll for a topic ( see below image)
you will be able to choose for each topic what level of notifications you want, even if you did not reply to it. To get notifications for all new posts choose the “Watching” level
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how to get notifications if a particular user posted a thread ? is there anyway to watch the user ?
stalking you mean 
hmm, no, I don’t think there is.
You can go a user’s activity page and see what he posted lately though.