1- I would say that you need a Problem, that is common to millions or billions of people around the world.
2 - You need to talk to those people to have a deep understanding of their problem.
3 - Iterate around the possible solutions for that problem.
4 - Show the solution (product or service) to the people that have the problem, and see if they use it, and if not why, and incorporate the learnings until you find a product/service that people want and would pay for.