The first thing you're going to want to do is set up a cluster in MongoDB. One way you can do this and easily visualize all of your databases, collections, and documents is through MongoDB Atlas. It's free, doesn't require a download, and allows you to easily connect to your cluster. You can quickly follow the first 3 or so minutes of this tutorial if you want more specifics re creating a cluster in MongoDB Atlas.
Once you have your cluster setup, you can go to the "CONNECT" option under your cluster. This should display three connection methods: choose the one that says "Connect your application." Here, you can whitelist your own IP address and then copy the connection string (URI).
There are other ways to set up the MongoDB database, such as through MongoDB Compass or other desktop applications, but the most important thing is that you have a URI for your database to connect to.
To set environment variables, create a copy of client/sample.env and server/sample.env. You can paste your MongoDB URI from above into server/sample.env, and the remainder of the variables come from Firebase.
Install all dependencies with the following:
npm install
This will install packages for both the front end and back end.
To run the entire application, use the following command:
npm start
from the root directory.