Execute a git pull in your repository directory to download new files. Prompt files won't change after posting, so feel free to modify them as you like. Here once again is an informal guide to git.
To develop efficiently, have a local web server running. Python has a simple, built-in server. Use python -m http.server for python 3; use python -m SimpleHTTPServer 8000 for python 2 (execute python in cmd for version number).
D3 is a library that makes it easy to create and manipulate page elements. It's not restricted to SVG elements, but that's what we'll usually use it for. We'll start by using d3 to select DOM elements, create new elements, and modify the content and appearance of elements.
You can find a bunch of D3 examples here and access D3's online API reference.
Here is a short guide on CSS selectors, which D3 uses for its "select" and "selectAll" functions.