you will create your first React native application. If you haven't set up your machine yet, please follow this article.

React Native has a built-in command line interface, which you can use to generate a new project. You can access it without installing anything globally using npx, which ships with Node.js. Let's create a new React Native project called "HelloWorld":
npx react-native init HelloWorld

If you have a physical Android device, you can use it for development in place of an AVD by plugging it in to your computer using a USB cable and following the instructions here.

If you use Android Studio to open ./HelloWorld/android, you can see the list of available Android Virtual Devices (AVDs) by opening the "AVD Manager" from within Android Studio. 

If you have recently installed Android Studio, you will likely need to create a new AVD. Select "Create Virtual Device...", then pick any Phone from the list and click "Next", then select the Pie API Level 28 image.

Click "Next" then "Finish" to create your AVD. At this point you should be able to click on the green triangle button next to your AVD to launch it, then proceed to the next step.

Running your React Native application
Run npx react-native start inside your React Native project folder:
cd HelloWorld
npx react-native start
The command above starts Metro Bundler (server)

If you are using your mobile device 

1. Enable debbuging over USB

(in the developer option in your mobile device)

2. plug in your device via USB.

To start the application run npx react-native run-android inside your React Native project folder:
Open new terminal
npx react-native run-android
then shortly the built version apk will get installed on your device or in your android emulator.

open the application to view your first react-native mobile app.