You need Node.js installed on your machine. Simply download the installer from nodejs.org and go through the installation steps.
Once Node.js is installed, open your command prompt or terminal and navigate into this project folder. There, run npm install to install all required dependencies.
Finally, run npm start to start the development server and visit localhost:8080 to see the running application.
Install the Firebase CLI globally: npm install -g firebase-tools.
Visit your Firebase project (in the browser) and download your security key file (Settings => Service Accounts). Store it in the functions/ folder and adjust your code in the function to grab that file.
Make sure to enter your Firebase project id into the .firebaserc file - deploy your project via firebase deploy thereafter.