The first step is downloading Android Studio. The link is https://developer.android.com/studio/index.html.
After that, you will be able to install emulator images. Please use the following guide to configure the AVDs (Android Virtual Device): https://developer.android.com/studio/run/managing-avds.html
Once you have installed some AVDs, you can see them on the CLI using the command: $ANDROID_HOME/emulator/emulator -list-avds
Start the emulator using the following command: $ANDROID_HOME/emulator/emulator -avd Nexus_5_API_25 -netdelay none -netspeed full
In this case, you need to replace Nexus_5_API_25 by an image that you have installed.
Now you can start developing by running this command:
react-native run-android