1. Introduction

Express.js is a fast, unopinionated, and minimalist web framework for Node.js. It's extremely popular because of its simplicity and flexibility. In this tutorial, you will learn how to set up a simple Express.js application.

By the end of this tutorial, you will:
- Understand the basics of Express.js
- Be able to set up a simple Express.js application

Prerequisites:
- Basic knowledge of JavaScript
- Node.js and npm (Node Package Manager) installed. If they are not installed, you can download them from here.

2. Step-by-Step Guide

Let's set up our first Express.js application.

Step 1: Initialize a new Node.js application.

Create a new directory for your project and run npm init -y to create a new package.json file. This file describes your application and its dependencies.

Step 2: Install Express.js.

Run npm install express to add Express.js to your application.

Step 3: Create an app.js file.

This will be the main file for your application.

Step 4: Set up your first Express.js application.

In the app.js file, add the following code:

const express = require('express');
const app = express();
const port = 3000;

app.get('/', (req, res) => {
  res.send('Hello World!')
});

app.listen(port, () => {
  console.log(`App listening at http://localhost:${port}`)
});

Step 5: Run your application.

Run node app.js to start your application.

3. Code Examples

Let's break down the code:

// This line imports the express module
const express = require('express');

// This line creates a new express application
const app = express();

// This line sets the port the application will run on
const port = 3000;

// This line sets up a route handler for GET requests made to the root path ('/')
app.get('/', (req, res) => {
  // This sends a response of 'Hello World!' when the route is accessed
  res.send('Hello World!')
});

// This line tells the app to listen on the specified port and logs a message to the console
app.listen(port, () => {
  console.log(`App listening at http://localhost:${port}`)
});

When you run this application and navigate to http://localhost:3000 in your web browser, you will see 'Hello World!'.

4. Summary

In this tutorial, you learned the basics of Express.js and set up a simple Express.js application.

Next, you could learn more about routing, middleware, and how to serve static files with Express.js. Here are some additional resources:
- Express.js Guide
- Express.js API Reference

5. Practice Exercises

Exercise 1: Create an Express.js application with routes that respond to '/about' and '/contact' requests.

Exercise 2: Extend your application from Exercise 1 to include middleware that logs the current date and time whenever a request is made.

Exercise 3: Create an Express.js application that serves static files (like HTML, CSS, and images) from a 'public' directory.

Solutions:

Exercise 1: Here's an example solution:

app.get('/about', (req, res) => {
  res.send('About page')
});

app.get('/contact', (req, res) => {
  res.send('Contact page')
});

Exercise 2: Here's an example solution:

app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();
});

Exercise 3: Here's an example solution:

app.use(express.static('public'));

With this solution, if you have a file named index.html in the 'public' directory, you can access it at http://localhost:3000/index.html.