Hello and welcome! That's a pretty open ended question that has a LOT of subjective answers, many of them with equal merit. You've got a few options for languages. Firstly, HTML, CSS, and JavaScript are going to be your front end; that's what browsers receive and create a web page out of. JavaScript, in particular, makes your website much more interactive, as opposed to click-button-reload-page.
On the back end, it gets more subjective. Your popular choices, in no particular order, are JavaScript, Python, PHP, Ruby, and many more. Personally, I recommend Python for beginners; it's easy to learn, relatively fast, very powerful, and can be used for most applications. JavaScript is also a good starting point; with NodeJS, the server-side of JavaScript, you can make a perfectly capable web service and it has the not-insignificant advantage that you'll have to learn fewer languages (since you'll need it for your front end anyway). Personally, I hate PHP; that being said, there are tons of applications built on it and it can do just fine. Ruby... has fallen out of favor, but is a viable language, as well. Again, it's very, very personal.
Both Python and JavaScript have a ton of resources to get started. I'd suggest building a "Hello world" application in both and see if either one clicks for you more than the other. What you're looking for is a "web framework," so that's what I'd start with in Google. I can't speak for JavaScript, but Flask is a great place to start for Python.
Unfortunately, I also can't speak for what it takes to get up and running on a Mac. It's closer to Linux, so it should be relatively easy. I know there's Homebrew, npm, and Anaconda that can make it much easier to install programming packages, but I can't test them myself.
If you have any more specific question, don't hesitate to create new posts. I'm pretty good with Python and okay with JavaScript and I try to stay active on here, but I, admittedly, have been slacking recently.
