I worked for 51 talk in China, teaching students at home as a contractor. How do I enter this income and deductions as it isn't a business, but has expenses?

If you want to report business expenses, you'll need to report your income as business income (self-employed) on the T2125 form. Don't worry, you don't need anything special to do this; Enter your own name and address as your business name and address and choice the industry code that best describes the kind of work you do.