Folders can be added under a list or under another folder and the way of adding in each case is very similar. Below another example of sample functions.
 
public SPFolder AddFolder(SPList parentList, String title)
{
    // Enable folder creation on the list
    parentList.EnableFolderCreation = true; 

    // Add folder
    SPListItem listItem = parentList.Folders.Add("", SPFileSystemObjectType.Folder);
    listItem["Name"] = title;

   // Update
   listItem.Update();

   // Return
   return listItem.Folder;
}

public SPFolder AddFolder(SPFolder folder, String title)
{
    // Add
    SPFolder newFolder = folder.SubFolders.Add(title); 

    // Return
    return newFolder;
}