Note: Make sure you have completed the React Native - Environment Setup instructions till "Creating a new application" step, before proceeding.
yarn install
cd ios && pod install
To start Metro, run the following command from the root of your React Native project:
yarn start
If everything is set up correctly, you should see the app running in your Android Emulator or iOS Simulator shortly provided you have set up your emulator/simulator correctly.
This is one way to run your app — you can also run it directly from within Android Studio and Xcode respectively.