In addition to having membership in the appropriate platforms' developer programs, submitting your application to either store requires that you provide it with an app logo, screenshots of the app, and a description.
Making your app look great
Creating an icon
Icons are the gateway to your application. It's what your users will interact with to access your app. To submit an application to the appropriate marketplace of your choice, it has to have an icon.
Because an icon is your app's brand, it's increasingly not enough to simply create a solid color shade and slap some text on it-users expect more.
Personally, I like using Sketch (https://www.sketchapp.com/) to design my app icons. While my design skills are not the greatest, I find it simple...