You can format text, add headings, images, tables, links, and other rich text formatting options if you enable Rich Text Formatting.
To enable Rich Text Formatting, go to "Create Content > Article" (or other content type).
Scroll down to the bottom of the "Body" input area. Below the input area you will see a link "enable rich text. Click on it. You will see a word-processor like interface, which many people will find more intuitive and easier to use.
NOTE 1: Rich text formatting is only available in the "Body" field.
In order to learn how to use each of the features, you have to experiment with each one of them. But most of the features are pretty much intuitive. In order to format bold text, highlight the text and click the button "B". Duh.
Some features are more complicated. But it's no rocket science, just try it and you will figure out how they work.
In order to add an image, click on the camera icon which reads "Add an existing Drupal image or create a new one". You will see a dialogue browse, from which you can either choose an existing image, or add a new one. You can also choose the size, and other options.