If you’re just getting started then HTML and CSS are absolute fundamentals for web development. These two languages aren’t really programming languages because they don’t deal in logic.
HTML is a markup language and CSS is a style language. HTML defines how a page should be structured and CSS defines how a page looks on the screen. These two languages are considered frontend languages because they operate in the browser(no server needed).
The two languages are very different, but they complement each other nicely. A page with just HTML will work, but it will not look pretty. Likewise, a pure CSS file actually won’t show anything in the browser because there’s no HTML to render.
If you can learn HTML and CSS then you can build websites. They may not be dynamic or programmable, but pure HTML/CSS websites can work and look good.
The other frontend language most web developers learn is JavaScript. With JavaScript you can build dropdown menus, sliding carousels, modal windows, and many other common web interfaces. More websites rely on JavaScript for dynamic features now than ever before. Modern web browsers finally support JS to a point where most users visiting your site will have JavaScript enabled. Get started with the fundamentals of JavaScript like setting variables, creating loops, functions, and logic operators. This knowledge can be applied to all forms of JavaScript and other programming languages too.
The last piece of web development falls into backend development. This is a broad area with lots of different languages, some more popular than others. PHP is one of the older web programming languages, and it was made specifically for web development.
Two other very popular languages are Ruby and Python. These are fun to learn because they’re programming languages that can be used beyond web projects.