Subscribing to a Forum
Enter the Forum and Select "Add Forum to Watched Forums" from the "Forum Options" dropdown.
Subscribing to a Topic
Enter the Topic and Select "Add Topic to your Watched Topics" option from the "Topic Options" dropdown.
Managing Watch Lists
Select "My Watch Lists" from the "My Stuff" dropdown, from here you can select the type of List that you wish to edit and by clicking the "Edit Watched Topics" (or "Edit Watched Forums" or "Edit Watched Users") link you can remove them or toggle the "Email Notification" option.
Toggling EMailed Watch Lists
See the above "Managing Watch Lists" directions.
Toggling the Default "EMail Watchedlist" Options
Select "My Preferences" from the "My Stuff" dropdown. From here you can manage the "By default should anything added to your Watch Lists be emailed to you?" option.