The easiest way to create your menu structure is to use WordPress Custom Menus.
Go to Appearance > Menus. There you can select all the pages you want in your menu, and you organize them as your like, including making some sub menus by indenting them to the right.
Then in The Manage Location tab, you select if your menu is primary or secondary.
If you need to learn more about this, you can search YouTube for WordPress Custom Menus Tutorials.