When we Google definition of word "app" we receive this definition "an application, especially as downloaded by a user to a mobile device" That is true but I have better definition "an app is downloaded content that provides user solution on problems that they are encountering".
It s not important if you work on Android or iOS platform . Getting idea is the same . We can search for idea ,but best idea find us.
First of all you need to create plan. To write good plan follow these easy steps.
I recommend YT Tracker developer @edouard_iosdev who is posting every day on instagram about his journey on the topic