By default Wordpress will show all published pages in your Primary Menu bar until you've created your own menu.

You can do this from Appearance > Menus in your WordPress dashboard. 

First, create a name for your new menu and click on Create Menu. Then select the menu items you want to add to the menu and click on Add to Menu. You can easily drag the menu items to change the order of appearance.

Finally, choose Primary Menu as the display location and click Save Menu. 

You can easily customize your menu design from Appearance > Customize > Header & Navigation > Primary Menu Bar. You can access a tutorial here.

NOTE: If you'd like to create a dropdown menu, please refer to this article.