HTML is the fundamental technology used to define the structure of a webpage. HTML is used to specify whether your web content should be recognized as a paragraph, list, heading, link, image, multimedia player, form, or one of many other available elements or even a new element that you define.
Ideally you should start your learning journey by learning HTML. Start by reading Introduction to HTML. You may then move on to learning about more advanced topics such as:
CSS, and how to use it to style HTML (for example alter your text size and fonts used, add borders and drop shadows, layout your page with multiple columns, add animations and other visual effects.)