It's very easy actually. These days there is not much difference between front-end and back-end as most of front-end is now written as applications (React, Angular, Vue.js) not webpages, same goes for mobile (React Native, Flutter). Since you are already a React Native Developer which means that you are hands-on on React.js (which is nothing but a wrapper on top of Vanilla JavaScript).
Gone are the days where back-end represented the meat of the application, even front-end has comprehensive logic now.
Here are the steps you should follow:
Since you are already using React Native (JavaScript), it will be very easy for you to pick up Node.js which is nothing but JavaScript on server. Make simple console application in Node.js first.
Now is the time to pick up a datastore. I recommend MongoDB, since you are dealing with JS and MongoDB has JSON-like documents.
Try to connect your Node.js application which you built in step 1 and fetch data from MongoDB and print it in console or write data from console to MongoDB.
Finally, just pick up a server framework like Express and start building API endpoints and consuming them in your front-end.
That's it. After you have mastered a back-end application it will be child's play to pick up another stack.
It's very easy brother. All the best :)