iOS
You need, at minimum, to learn Swift programming language, Apple computer (macbook pro or desktop) with Xcode IDE, and some design skills for building interfaces and graphical assets. You will need an annual $99 developer license to publish apps. Also, while not required, you should have an iPhone, iPad or other similar device to test your applications on a real device.
Android
You should learn Kotlin or Java programming language. You also need Android Studio IDE and a computer that can run it; can be windows, macOS, or linux. The more RAM and storage you have the better, because debugging Android apps is memory intensive and emulators require a lot of storage space. You will need to be able to design interfaces and graphical assets and necessary software to accomplish that task. You also need to purchase a one-time license for $25 to publish apps in the Google Play store. Having an android device is optional and very useful, but not required.
Other frameworks
Several cross-platform options exist, such as React Native, Xamarin, Flutter, etc. The benefit of these platforms is the ability to build applications for iOS and Android simultaneously, but you are then making your application dependent on the framework. The hardware requirements for these frameworks vary.