Need help customizing Spirit theme, please

as a complete newbie to WP and a relative newcomer to php, I’m failing dismally in my attempted customization of this theme. I’m trying to use WP as a CMS for the site https://aolff.org/wordpress.

What I’d like to do is have my navigation across the top list all of my pages.

Each of those pages is going to have a varying number of sub-pages beneath them – when the main page is clicked, I’d like the subpages to appear in the side menu.

Can anyone point me in the right direction to make this happen?

Thanks!