Here's a collection of guidebooks and tutorials that give the basics of HTML, CSS and JavaScript – three languages that form the basis for the modern web. The Mozilla links are documents that you can read through. The CodeCademy and other sites are interactive tutorials to get used to writing code.
For future projects, install Visual Studio Code, which is a handy text editor, useful for more than just web development (you can take interview notes in it, for example). Also, you'll want to install XCode on your mac, which you can access via the App Store on your computer. It's a big download (around 3gb) so it takes a while.