To add a section to your site, first choose a group to add it to. If the group is empty, you can click the “Add Section” button in the middle. If not, you can click the plus to the upper left of the group when you hover over that group.
Once you have clicked either of these buttons, you will be presented with a list of different section types. Choose the most appropriate one for your content. Most sections will be “Plain Text” type which is why that option appears first. You can learn more about the various section types in the Content Sections area of these instructions.