Start with a terrific idea/concept that should be big enough to make a difference, small enough to be achievable and something you believe in, something you’re willing to dedicate your life to, and it should be something that will attract the interest and energy of others.
Next consider the market size and whether the market is large and growing and does the market have a demonstrable need for the product or service, and will the company the market driven, or basically will anyone care about the product and the fact that it is available.