What should I learn to make an app for both iPhone and Android?

React Native is a reasonable first choice if you want to build on your JavaScript knowledge. You can use JavaScript or TypeScript, and the component-based approach may feel familiar if you have used React.

One caveat: knowing JavaScript does not automatically mean you know React. Learn the basics of components, state, and navigation, then make a small app before committing to a bigger project.

Think about the app you want to make, not only the language. A basic app with forms, lists, and network requests is a good fit for a cross-platform framework. If it needs lots of platform-specific features or very custom interactions, expect to learn some iOS and Android details whichever framework you pick.

For a beginner, I would choose one framework and finish a small project rather than spend weeks comparing every option. React Native is a practical match for your existing JavaScript experience.

Also look at Flutter if you are open to learning Dart. It is another cross-platform route, but it will mean starting with a new language. The best choice depends on whether keeping your JavaScript skills in the loop matters more than trying a different toolkit.