A couple of things you need to install for setting up the environment. If you want to build an iOS app, you need a Mac OS and to build an android app, you can go for Mac orLinux or Windows OS.
iOS app
Mac
android app
Linux
Windows
Step 1: Install Node
Node
Step 2: Install React Native.
React Native
npm install -g react-native-cli
Step 3: For Android App - Install Android Studio and Android SDK For IOS App - Install XCode
Android Studio
Android SDK
XCode