For individual topics, with the topic open, click the Notify button, which is the 2nd button to the right of the Reply button.
To be notified about new posts in specific boards, with the board open, click Notify button for that board, which is the 2nd button to the right of the New Topic button.
For global notifications, go to Profile tab > Modify Profile > Notifications. There are a few different options for setting that up.
Just a friendly reminder -- If you get email notifications, and you reply to that email, no one will ever see it. You have to post replies in the forum.
