Which topics should I add to my community?
That depends. If you support several products, you might add a topic for each product. If you have one big product, you might add a topic for each major feature area or task. If you have different types of users -- end users and API developers, for example -- you might add a topic or topics for each type of user.
A General Discussion topic is a place for users to discuss issues that don't quite fit in the other topics. You could monitor this topic for emerging issues that might need their own topics.