Before doing anything else, create some file folders, which you will use for your images, template and css files. You can create these folders in TYPO3. Click on "Filelist" in the "File" menu, click on the "fileadmin" folder and use the green plus sign to create new folders. In the "fileadmin" folder, you create 2 new folders called "images" and "templates". In the "templates" folder, you create 4 new folders called "css", "images", "layouts" and "partials". The "layouts" and "partials" folders will only be used if you choose Fluid as your templating solution.
The first images folder will be used for images which are part of the content. The second images folder will be used for images which are part of the template (backgrounds, header images). You'll end up with this:
You can also upload files in the file list. Click on the folder where you want to upload files to and then click on the page-with-green-arrow icon (next to the green plus, on the left top of the editing window).
Please note when using Fluid: modern tutorials recommend placing the Fluid template files in an extension. That's far better than putting them in the fileadmin folder, as the fileadmin folder is publicly available. An extension is not (or it shouldn't be), and it can be exported and imported, much like a theme. Please refer to the TYPO3 documentation about this. In this tutorial we'll use the fileadmin folder for educational purposes. It's also easy to be able to update template files from the CMS when you've made a mistake.
Please note when using Templavoila Plus: you don't need to create the layouts and partials folders. Furthermore, storing the template file in the fileadmin folder is required for Templavoila.