It is easy enough to learn React Native if you know react already. The difficult parts are largely the same. You have to get used to it because styling and UI/UX is pretty different for mobile applications. But, if you know React well you need only a couple of days to transition to react-native.
If you are interested to learn React Js, check out this React Js Training Course by Intellipaat.
Also, check out this video for more similarities and differences between React.js and React-Native: