Great advise for creating a product (web app) on the web, which is supposed to make money:

Go with the market-first approach as opposed to idea-first approach. If you are confident that you will be able to make a good enough product/prototype, I suggest taking the following approach:

• Find an industry (ideally, an old fashioned one) where people are making money
• Find the single differentiator which will put your app apart in the already established industry (read or research what pain points are still not addressed by the top 3 solutions)
• Make a web app, market it, refine it based on feedback, and monetize the app
• Slowly incorporate all standard features expected from a solution in that industry so you can shoot to becoming a market leader