This section explain how to build signed apk for both the iOS and android devices

For android

Follow the instructions here to build a signed apk https://facebook.github.io/react-native/docs/signed-apk-android.html

You can ignore the key generation because there is a key already bundled in your download just scroll down to the section for generating the apk

$ cd android

$ ./gradlew assembleRelease

For iOS

Follow the instructions here to build for production in your XCode here https://facebook.github.io/react-native/docs/running-on-device.html#building-your-app-for-production

Note : You will need to have an account with apple by paying the $99