You need to build a "low fidelity" web or mobile app in order to gain customer feedback.

Low fidelity means something unfinished (wireframe, basic site etc). The point is to get as much feedback and insight into your understanding of the problem and how you present the solution to the customers.

Later on, once you've gathered lots of feedback you'll build a "high fidelity" app to test your understanding of the solution.

This process helps you avoid building products no one wants.