I would definitely start with React.js which is used for web app development. You can also make iOS and Android apps using React Native and desktop apps using Electron. Because these frameworks are 95% similar to React.
So by learning one framework and one language (JavaScript) you managed to cover all these different platforms. And that is truly powerful.
Not to mention JavaScript can be also used on the backend to develop server-side apps.
Overview
React.js framework is used to build web app interfaces. And it helps developers to structure their code in a way that is scalable.
React Native follows almost the same structure as React.js and allows you to build native apps with JavaScript. So by knowing React.js you can immediately build iOS and Android apps.
Electron framework is again similar to React.js and it allows you to build desktop apps without having to learn any new language or framework.
And that is why starting with React.js is your best long-term decision.
P.S. -> read this article if you want to double your salary