You need install Node, the React Native command line interface and Xcode to build ios application or Jdk and Android Studio to build android application.

1. Move to Homebrew then install Homebrew
2. Run the following commands in a Terminal after installing Homebrew:
   brew install node
   npm install -g react-native-cli
   brew install yarn
3. You need Xcode to buid react-native iOS application.
   The easiest way to install Xcode is via the Mac App Store. Installing Xcode will also install the iOS Simulator and all the necessary tools to build your iOS app.
   
4. You need Android Studio and Jdk to buid react-native android application.
   Download and install Oracle JDK 8
   Download and install Android Studio
   More information about install and build iOS, android app Getting started with react-native