The main component of most articles is probably text. Besides text, you can also add various other components. You can learn how to do that in this article.
Normally, when you create an article, you use a template. This template already has a text area predefined. To write in this area, you simply click in the editor on the text field where you want to write. With a single click you select it, with a second click you can start writing. You can see that the cursor inside the text field is blinking.
In addition to normal text you can also insert other components like images. You can learn how to do that here. If you want to add more text, you have to add a new text component. This works the same way as for all other components (read here if you are unsure). After that, click twice on the text component again, so that you can continue writing.
When you are done with the text, you can save the article via Save and publish it directly via Publish.