The most powerful words to find your niche to nail are 'best' and 'only'. When you find something that 'only you can do' or that you're 'best at' in a specific part of the market, you should explore that segment of the market.
Before you can succeed in creating your positioning, messaging, branding and marketing execution, you need to establish your core identity. What your best at? What can only you do? For who?