The first thing you need to decide is what type of app you want to build: native, hybrid, or web. Native apps are written in the platform-specific languages and frameworks, such as Swift or Objective-C for iOS and Java or Kotlin for Android. They offer the best performance, functionality, and user experience, but they also require more development skills and resources, and you need to create and update separate apps for each platform. Hybrid apps are written in web technologies, such as HTML, CSS, and JavaScript, and run inside a native container or wrapper, such as Cordova or Ionic. They allow you to use the same code base for multiple platforms, but they may have lower performance, limited access to native features, and less consistent user interface. Web apps are also written in web technologies, but they run in the browser and don't need to be installed on the device. They are the easiest and cheapest to develop and update, but they have the lowest performance, functionality, and user experience, and they depend on the internet connection and browser compatibility.