What are you really good at? This may be your professional area (e.g., design, marketing, or medicine) or an area of your personal interest (e.g., beauty, traveling, or healthy lifestyle), but it should certainly be something you are passionate about. If you're bored, your audience will be bored as well.
A niche could be an area of your expertise, something you can share information about right off the bat. But it doesn't have to be. Many people around you are doing their first steps in the same area, and you can do it together. You can demonstrate how you learn a topic, show your first results, and share your progress in becoming an expert.