Create a new folder in your Drive, and name it the same name as your new Google site.
Right click on the folder, and go to 'Share'. Check that the permissions are set to 'Anyone with the link can view'.
Now use this folder to store anything that you put onto your Site e.g. images, videos, docs, etc. Everything you put into this folder will inherit the same permissions as the folder, and will be visible to everyone.
Choose a look for your site. Each theme comes with a preset background, color scheme, and font selection. You can adjust fonts, colours, and the background later, and you can always change the theme after the site is created. If you need to make any changes, click Undo undo, or Redo redo.
Remember to click PUBLISH when you have finished, or else no-one can see your changes.
Keep viewing your Published Site to see what the world will see.