What
What you are looking into is 'web development', which, if you want to specialize, is divided into two areas-- "Front-end development and back end development". Front-end development is client-side, meaning it deals with the browser oriented side of development. Back-end development deals with server-side development. To elaborate on front-end development, it typically deals with how a page looks, interactivity on the page, etc., without referencing a server (back-end development). Front-end development deals with HTML, CSS and Javascript. HTML is used to give a page structure, CSS is used to give pages style and Javascript can be used to give a page behavior/interactivity.
Back-end development involves using languages like PHP or frameworks like Ruby on Rails (Ruby), Django (Python), asp.net, etc. In conjunction with knowing a 'server-side' language, you will also have to learn some sort of database, which will likely be an SQL variant of some sort.
If you are getting into web development, learning HTML and CSS is critical, since whatever language you choose afterwards will either be generating HTML or manipulating HTML elements. For example, with PHP or some other server side language, the user will do things like submit an html form, the results will be sent to a server on which your script is stored... your script will process the results and return the results to the user. With front-end development, which is Javascript, you will be manipulating HTML elements for some purpose or the other. For example, when the user clicks on a button, it will run some javascript code to react based on how you want it to react.