First run npm i to install all dependencies. You may also need to do this in the client-react directory (since create-react-app creates its own package.json with its own dependencies).
Run mongod --dbpath=./data to start the database. This assumes you have created a directory called data (it won't be there when you clone the repo because it is in the .gitignore file.
Run npm start in the root directory to start the server.
To start the fronted, cd into client-react and then run npm start.