If you are solving a big problem through your startup then people will love you and will definitely use your app. Necessity is the mother of invention and failure is the mother of success. So don't be afraid to fail, just build it and present it to the consumers.

For a massive launch, you have to do Growth Hacking. It's a combination of engineering and marketing. This is the practice that is common in leading startups for the app launch.

I am sure, I can help you if you wish to setup a call. I am an expert with multiple skills :)

My best wishes.