Before you can start programming, you need to install the Python interpreter. Think of this as the engine that reads and runs your code. We'll be using Python 3.5 for this course.

First, head to the official Python website at python.org. Navigate to the downloads section and look for Python 3.5. If you're using Windows, you'll want the executable installer. For macOS or Linux, you'll grab the appropriate package for your system.

When you run the installer, you'll be presented with a few options. The default settings are usually fine, but there is one crucial step you can't miss.

If you forget this step, you'll have to configure your system's PATH variable manually, which can be a bit tricky. Checking the box handles it for you automatically. Once you've done that, proceed with the standard installation.

Once the installer finishes, it's a good idea to confirm that everything is working correctly. You can do this using your computer's command line interface. On Windows, this is the Command Prompt or PowerShell. On macOS or Linux, it's the Terminal.

Open a new command line window and type the following command, then press Enter:

If the installation was successful, your computer will respond with the version of Python you installed.

The x will be a specific number for the minor release you downloaded. If you see this, you're all set. If you get an error message like "command not found," it likely means Python was not added to your system's PATH during installation.

Python comes with a built-in tool that lets you run code one line at a time. It's called the interactive shell, or REPL. This is a great way to experiment with code snippets without having to save them in a file first.

To open the REPL, just go to your command line and type python with nothing after it, then hit Enter.

You should see a new prompt, which usually looks like three angle brackets (>>>). This means Python is waiting for your instructions. You can type a simple command, like a math operation, to see it in action.

Python reads your input (2 + 2), evaluates it, prints the result (4), and then loops back to wait for your next command. That's the Read-Eval-Print Loop. To exit the REPL and return to your regular command prompt, you can type exit() and press Enter.

Now that you have Python installed and know how to access the interactive shell, you're ready to start writing code.