If your OS is:
Linux. You already have everything. Make sure to install g++ if it is not already there.
OSX. You already have everything. It may come with g++ pre-installed. However, if it’s not the case, install Xcode or its Command Line Tools. This link may be helpful. Open the Terminal and you are good to go.
Web-based. If the above options don’t work for you, there is a good online development environment.
For this class, I strongly discourage using Xcode or any other IDE. We need only a text editor and a terminal window.
If you are using Vim (and Vi) text editor: The editor does not come with a good default configuration. Please refer to this quick guide to make it suitable for editing code.