React is a Javascript library used to build user interfaces. As such, you're going to have to know HTML, CSS and Javascript in order to learn React. It's ease of use and popularity comes from how little you actually have to code. Easily applicable for any frontend interface, if you're going to use any library/framework for frontend development, React is the safest option.
These frontend challenges for HTML, CSS and JavaScript will help you review core skills while also making interesting projects as well.
https://www.frontendmentor.io/
Basic React tutorial provided by the official React website. This tutorial will guide you through a very basic tutorial that you can do straight from your browser.
https://reactjs.org/tutorial/tutorial.html
React tutorial by MDN Web Docs. Useful readings for if you have time, but not necessary by any means.
https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_getting_started
Javascript frameworks tutorial provided by Microsoft. Useful readings, but not necessary to learn React.
https://learn.microsoft.com/en-us/windows/dev-environment/javascript/
1 hour in depth tutorials about React. These Youtube crash course provides a great in depth look at React and the various important components that you will be using most often. Very useful if you are learning React under a strict time limit for whatever reason.
https://www.youtube.com/watch?v=w7ejDZ8SWv8
https://www.youtube.com/watch?v=b9eMGE7QtTk
Online development environment to try React out in (Among other frontend tools). Great for testing the waters with React to see if it will be something you want to learn or not.
https://codepen.io/
Host your React websites straight from your Github repo. Website hosting is fairly expensive, so this is here as a free alternative for website deployment.
https://pages.github.com/