TYPO3 Basics: Basic file folders
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).