To create a simple "Hello, World!" application in React.js, follow these steps:

1. Set Up Your Development Environment :

   Make sure you have Node.js and npm installed on your system, as described in the previous response.

2. Create a New React App :

   Open your terminal or command prompt and run the following command to create a new React app:

       npx create-react-app hello-world-app

    This will create a new directory called `hello-world-app` with a basic React application structure.

3. Navigate to the New Project :

   Use the following command to navigate into the newly created project directory:
     cd hello-world-app

4. Open the App Component :

   In the `src` directory of your project, you'll find a file named `App.js`. Open it in your preferred code editor.

5. Replace the Content:

   In `App.js`, you'll see the default content. Replace it with the following code:

 
   import React from 'react';

   function App() {
     return (
       <>
         <h1>Hello, World!</h1>
       </>
     );
   }

   export default App;
 

   This code defines a functional component called `App` that returns a JSX element containing a heading with the text "Hello, World!".

6. Run the Application:

   In your terminal or command prompt, make sure you're still in the `hello-world-app` directory, then run the following command:

      npm start

   This will start the development server and open your default web browser with the React application running. You should see "Hello, World!" displayed on the page.

  Congratulations! You've just created your first React.js application.

You can now continue building on this foundation by adding more components and functionality as needed.