Please follow instructions on React Native CLI Quickstart tab on Setting up the development environment page to setup your development environment https://reactnative.dev/docs/environment-setup
This is important step, so please read carefully and follow the doc step by step.
We recommend you using Yarn for package manager. From macOS you can use the command bellow to install Yarn throw the Homebrew package manager ( installed when you done development environment setup process ). Read more details at: https://classic.yarnpkg.com/en/docs/install
brew install yarn
When done, let create new application and test your development enviroment