As Ronald's suggest create something more than a todo app with it.
As you proceed and start to work on more complex apps, you will need to manage state effectively. You can take a look at MobX or Redux.
You will also want to understand how to do routing inside a React app. Take a look at React Router.
React community also have tried out CSS-in-JS flavors – styled-components, glamorous, css-modules. These ideas shed some new light on how we use CSS.
As most frameworks, React has its own essence. Its a tool to create better abstractions to express your app. A lot of focus these days in the React community is composition of components. In order to understand these advanced patterns, you would want to understand the basics concepts of React – react elements, react components, reconciliation.
React 16 has some new ideas worth exploring too.
After spending a decent time with all these, if you feel interested you can discover if static typing tools like Flow or Typescript will help you write more robust code.
My intention is not to come up with a syllabus to learn React. Instead, I would like any new comer to acquaint themselves with the ecosystem and learn what is needed, when it is needed.
React is an ecosystem to express UI interactivity. So, there is a lot. My suggestion would be not to rush. Take one thing, learn it, enjoy it(create small demos) and try to realise how it fits in the ecosystem. If its possible, you can make use of good paid content out there too.
A lot of the ideas helps you understand the UI paradigm better, it transcends the framework. The benefits are beyond just learning React.