You can create apps for Android as well as iOS platforms. Both platforms have different build, so the procedure of creating an app is also different for both platforms. Here are the five steps that are to follow to create an app:
Building an iOS App is the process of creating an app to making it available to the users by publishing it. You release your application through an application marketplace, such as the AppStore. When you publish your app on the AppStore, you put your app in front of the immense active customers of the AppStore. For the complete procedure, refer to Building an iOS App.
Building an Android App is the process of creating an app to publish it that makes your applications available to the users. You release your application through an application marketplace, such as Google Playstore. For the complete procedure, refer to Building an Android App.