Here at IFB we always advise you to focus on a niche. But even more than that, to focus on a niche that you are passionate about. How do you find out what you are passionate about? There is no self-help quote that can help you, except the quotes that say to follow your instincts. Pay attention to what topics bring you joy, that make you happy, that inspire you to create more.
Write about what rings true to yourself. That will give you a sense of authenticity that can't be replicated, and ultimately lead you to write more about what you love.