To add a menu to your Drupal site, you should enable menu module from your modules page:

 

When enabled, head to admin/structure/menu/manage/main-menu and a few menu links:

 

Depending on your theme structure, you have two options, either programmatically call menu in your theme, or put it on block. Since in this tutorial we are using Bootstrap theme, which has already called function to show menu, when enabled, after adding menu links, they will be visible on Bootstrap theme.