how to make the header sticky?

Try adding some custom CSS like this. You can adjust 100px to whatever fits best with the height of your menu bar.

.site-main .content-area .site-content, #secondary {
     margin-top: 100px;
}

Don’t edit the theme files directly, otherwise your changes will be overwritten when the theme is updated.

An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.

As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.

If you need further help, please provide a link to your site. Thanks.