How to add content to the menu

Use Appearance, Menus in the Admin area to add menu items; create a menu first if not already there.

Then assign the menu to the site via the Manage Locations tab on the same screen. Depending on the theme you use, you’ll be able to assign your menu so it appears where allowed by the theme.

You can assign pages/posts/custom links/categories to your menu on that screen.

You’ll find lots of videos on the subject if you ask the googles..there are also some good ‘beginner’ sites: I like www.wpbeginners.com (not affiliated with them). Just keep looking around on the sites or the googles to figure things out.