A couple of questions regarding server-side programming.

Basically to get started with Express.js (Node.js presumably) I would suggest Express Application Generator. This is the easiest way to get started with a skeleton of an Express application and will give you all the basic structure for a Hello World application in Express.js.

To have the user login with Facebook or Google or Twitter or pretty much anything, use Passport.js. This is a really extensive library and provides much comprehensive options for authentication.

Now about the front-end server. It is the server usually dedicated to handling the requests from the client and responding to it. Handling bad requests and validations as such. Of course, it being named front-end, it is facing front end but not that it is on the client side. Usually this term is used when you want a scalable application and you want to use something called Message Passing Mechanism to make your application scalable. In this you divide your server into two parts. One handles the requests from the client (User) and it communicates to the message passing mechanism for any business logic or processing. The other part reads the messages from message queues and provides services to process the messages according to the intended business logic. Here, the first part, handling requests and requesting the business processing, is the front-end server. And the second part doing the business processing and communicating to the database, etc is your back-end server. To understand the message passing architecture more, checkout Wikipedia pages of AMQP and RabbitMQ or Kafka. For your application though, I would recommend RabbitMQ. Also, visit RabbitMQ official website and documentation. It is really well written.