Creating a Sub-Menu Item on your Main Menu

First, create your new article. This will be your new page. Then click Save & Close

Next, click "Menus" from top menu
Click "Main Menu"
Click green "New" button in upper left corner
Add Menu Title (this is what will display on your menu)

Menu item type: Click blue "Select" button
Click "Articles"
Click "Single Article" (last option in Articles options)
Click "Select Article" (gray button)
Click title of your new article

ON THE RIGHT HAND SIDE
"Parent Item" - select from drop down the space you want your new page to display underneath in your main menu
Click "Save & Close"

Go to your page & refresh, test your new menu item!