At first check if mongodb is installed in your system. Go to backend folder in command line or terminal and type
npm install
After that type
npm start
after that
cd..
cd ..
Go to the https://facebook.github.io/react-native/docs/getting-started.html
Learn how to install react native in your computer or system under building project with Native code tab
After the installation of react native in your system
Type:
npm install
or yarn add
in the project directory where you downloaded the source code
After that:
react-native run-android