Getting page ID in order to make submenu

This might seem elementary but I’m grappling with WP for the first time (though I’m used to PHP and MySQL).

I want to make a sub-menu dependent on the page being viewed so I’m trying this:

<ul><?php wp_list_pages('title_li=&depth=1' ); ?></ul>

<ul><?php wp_list_pages('title_li=&child_of='.this_page_or_its_parent_here().'&depth=1' ); ?></ul>

I’ve tried all sorts but can’t find a function for
this_page_or_its_parent_here()
shall I just write it? Surely it exists already?

Thanks in advance,

Pete