Hello!
There are definitely topics about this out there, but I’m not sure if there is one that concisely answers your specific question.
The site setting you’re looking for is default categories watching first post. Adding categories to that setting will cause all users who sign up after you modify the setting to receive notifications (and thus emails) for new topics in each category you add to that setting. If you want users to receive emails for all new topics on the forum, add all the categories (and subcategories). If you want to set specific categories. only add those.
Keep in mind, users will only receive emails for new topics, not any subsequent replies. If you’re looking for users to receive replies by default too, add categories to the default categories watching site setting instead.
Lastly, if you have existing users and want to modify their preferences, see How do I set category tracking level defaults historically.