I have installed create-react-app exactly as instructed on the facebook instruction page (https://facebook.github.io/react/blog/2016/07/22/create-apps-with-no-configuration.
I hope you already installed Node package manager(npm).
now run npm install -g create-react-app, if everything fine then you can use create-ract-app command.
if you are getting any permission error just sudo npm install -g create-react-app.