The hardest thing for me was learning how to teach ground school. Now, teaching in the airplane was very natural for me because I am a 'hands-on' type of person.
It's very hard to know everything about every topic because our knowledge base is so broad. So, if you don't know something, do NOT B.S. the student. Turn their question(s) into a research project. Either look it up together, or make it a homework assignment, and have THEM teach you about the topic they had a question on.