Setting up

• Node JS
• Flow
• Watchman
• Sublime Text 3 or Atom is good to start

More, if you want iOS

• Xcode
• iOS Simulator

More, if you want Android

• Android SDK
• Android Emulator (React Native suggested Genymotion)

You needs these skills to go well:

• HTML, CSS
• JavaScript
• ES6
• JSX
• Basic using of Xcode
• Basic using of Android Studio

For  setting up react native  into  your development environment follow the official tutorial