In order to create onepage site you need to connect navigation menu items with different page sections.
Page section is a row element with its own ID. While you add a new item to your menu, you need to set an anchor to the particular page section (row) using that ID.
See the screenshots below, they show the example of adding a section to the menu while creating a onepage site.
1. Setting an ID for the row / section with proper content.
2. Setting an anchor to the section for menu item.
In result you get smooth scrolling to the particular section after clicking on appropriate menu item.