Build a dinamic sub-menu in custom theme

HI guys!

I’m building my first theme and i don’t knew, it’s so easy! But… i have a question.

I added new pages, and I added sub-pages (sub-menu). But they aren’t visible on CSS.

There a function that get the class of menu for build a sub-menu on hover in my custom theme?

Example:

<ul class='sub-menu'>
<li class='menu-item-89'>submenu</li>    <-- i want get this class element to show a sub-menu dynamically
<li class='menu-item-88'>submenu</li>    <--  i want get this class element to show a sub-menu dynamically
<li class='menu-item-87'>submenu</li>    <-- i want get this class element to show a sub-menu dynamically
</ul>