To create a folder select the action "New folder" in the sidebar of the file area. A dialog will open where you can specify the new folder's name and its description and select a folder type. Depending on the file area (course file area, institute file area or personal file area) different folder types are available. The selected folder type influences the behavior of the folder and which permissions other users may have on the folder's content.

After you click "Create" on the bottom of the dialog the folder will be created.