To start building on Gutenberg (or almost any other page builder), you have to first conceptualize the idea of content blocks.
Instead of a constant flow of paragraphs like many other website builders do, each “line” is actually a block, which can be an image, a paragraph, a column, or a form.
When you hit Enter in a block editor, you’ll see a black square with a plus sign in it. Click on it and it’ll open a menu with the available blocks. Alternatively, you can simply type in the slash command to open the menu (/).
From there on, you can simply start adding, moving, and removing blocks at your will.
As mentioned, certain blocks can contain other blocks.