Hi
Unless you're doing something really unique (and even then...) it is actually advised that you start with something that already exists in order to first test the market and see that your idea/business is indeed feasible. Once you've validated your idea, you can spend money (if need be) on creating something tailor made.
By testing a simple platform first, you will learn a lot about the market, the users, what things you should do different etc....
I am a mentor on Startups.com, a lecturer, and a startup lawyer. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you. After scheduling a call, please send me some background information and the 2 main questions you want answered so that I can prepare in advance (to give you maximum value for your money). My reviews: https://clarity.fm/assafben-david