I will not go into too much details but I want to mention that store optimization is a good place to start but it will not get you too far.

High quantity of downloads and reviews gets your app noticed.

The more downloads and reviews your app gets, the more people will think: "If that guy (girl) liked it so much - let me download it 2". And then the app gets the 'snowball' effect - keeps getting bigger and bigger. But you need to promote the app.