The interesting part is that you don't have to go know all the courses in this world to be able to make money. If you are a science student, you can choose to teach any of the stem courses. Mathematics especially, everybody wants to know maths because it's a must. Whatever courses or subjects that requires calculation requires you to know math, it's like the basis. You will have to study this subject very well and get familiarized with most of the questions. You can decide to organize a physical class or offline class. Whichever one you decide, you can drive traffic from it. Those you teach online might want to have a physical class with you, those you teach physically might want to further by following you up online, they can even give recommendations too. When they discover how good you are, they would be willing to give more recommendations and pay you more money.