To add a secondary menu (top bar menu), you need to visit the Appearance » Menus page in your WordPress admin dashboard.
Click on Create a New Menu
Enter the Name of the menu (ex: secondary menu),
Click on Create Menu
After that check each of the pages you want to add to your menu, and then click on the Add to Menu button.
Once your pages have been added, you can move them around by dragging and dropping them.
Note: All menus have their items listed in a vertical (top to bottom) list in the menu editor. When you put the menu live on your site, it will either display vertically or horizontally (left to right).
After adding pages to the menu, select the Secondary Menu in the menu settings, and click on the Save Menu button.