Like the Skilled Seducer forum?
It's built on Xenforo, which is a turnkey solution. However, we also have a custom theme and custom functionality. For that, you need designers to design the look of the theme; you need someone who understands CSS and HTML to code the actual theme; and you need someone who understands PHP to build the custom functionality.
If you mean the main site (Girls Chase), it's built on Drupal. So you'd need all that (CSS, HTML, PHP), plus things like SCSS and DOM structure. Plus Drupal is its own environment, so you'd need experience working with Drupal as well.
We're in the process of upgrading to Drupal 9, which would require you to be familiar with Symfony too.
That's not including the backend, systems administrator stuff. Our lead Drupal dev knows a lot of that stuff as well; command line, server-side optimization, and so on. However, we also have a dedicated sysadmin who is very good.
We use git for version management. Which is definitely a Godsend because back in the day I used to go in and make changes to the site directly without version management, and if you screw something up it can take you 18 hours of backtracking and swearing to yourself trying to undo it. With git you just push a button and the whole thing reverts to how it was before the update.
Then on top of that there's the various caching, proxy stuff, etc., to make the sites load faster.
You've got databases, which it's useful to know how to work with; if you're working with them it's worth being competent in MySQL commands. I had a class on this in college that came in handy as I've occasionally had to go into one or the other of our DBs to run queries and make changes myself.
Anyway -- yeah, that class if it's getting you familiar with HTML and CSS will give you the basics of web design. That would allow you to start playing with a simple Xenforo forum, or a Wordpress or Drupal website, and making some changes on your own.
For any of those you'd need PHP for custom module/plugin/functionality development. But HTML/CSS are a good basic first step.
All my first websites were simple hand-coded HTML/CSS projects. It's as good a place to start as any.
Chase