To Subscribe to any topic, i.e. get an email when there is a new post just click on the "Subscribe topic" link at the end of the topic. To unscribe just click on the "Unsubscribe topic". You can also set your default in your User Control to automatically subscribe you to any topic you post to. The default is 'no'. You can also manage your topics subscribed to there.
Read the FAQ. It answers many questions and shows you some of the finer points.
What is the difference between bookmarking and subscribing?
Bookmarking in phpBB3 is much like bookmarking in your web browser. You aren't alerted when there's an update, but you can come back to the topic later. Subscribing, however, will notify you when there is an update to the topic or forum on the board via your preferred method or methods.
I haven't had time to try the Bookmarks feature yet so I'm not much help.