To get data from a backend (such as a server or API) to your React.js application, you typically use AJAX requests or fetch API to communicate with the backend. Here's a general outline of how you can achieve this:

Set up your backend: You need a server or an API that your React.js application can communicate with to retrieve data. This backend could be built using technologies like Node.js, Express.js, Django, Flask, etc., depending on your preferences and requirements.

Create API endpoints: Your backend should have API endpoints that the frontend can call to retrieve data. These endpoints could be RESTful APIs or GraphQL endpoints, depending on your application's needs.

Make HTTP requests from your React.js application: In your React.js application, you can use the built-in fetch API or libraries like Axios to make HTTP requests to your backend API endpoints.

Handle the response: Once the backend responds to the request, you need to handle the response in your React.js application. You can then update your application's state or perform any necessary actions based on the received data.

Here's a simple example of making an HTTP request using the fetch API in a React component:

javascript
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import React, { useState, useEffect } from 'react';

function App() {
  const [data, setData] = useState([]);

  useEffect(() => {
    fetchData();
  }, []);

  const fetchData = async () => {
    try {
      const response = await fetch('https://your-backend-api.com/data');
      const jsonData = await response.json();
      setData(jsonData);
    } catch (error) {
      console.error('Error fetching data:', error);
    }
  };

  return (
    <div>
      <h1>Data from Backend</h1>
      <ul>
        {data.map(item => (
          <li key={item.id}>{item.name}</li>
        ))}
      </ul>
    </div>
  );
}

export default App;
In this example, the fetchData function is called when the component mounts (thanks to the useEffect hook). It makes a GET request to https://your-backend-api.com/data, parses the JSON response, and updates the component's state with the received data.

Remember to replace 'https://your-backend-api.com/data' with the actual URL of your backend API endpoint.

This is a basic example to get you started. Depending on your application's complexity and requirements, you may need to handle things like error handling, loading states, authentication, and more.