What will it look like? Although it’s a little too soon to be calling a graphic designer, you should start to think about how the app will basically look, how the user will interact with it, and how it will flow from one screen to the next.
Sketching wireframes is a great way to help you further refine the idea, and get a better feel for how the app should function.
List the main screens that the app will have and sketch each one, showing the rough layout of each key component. Title each drawing and write notes for all of the interactive parts, saying which screen the user would go to next if they interacted with those parts.