Each main category should serve as an umbrella for the content you will be adding. Under those topics, you will probably have subtopics. They will appear in the sidebar menu and under the main topic in the drop down menu. These subtopics need to be highly related to the main topic you place them under.
1. In the admin toolbar, hover your cursor over Content.
2. Pull your cursor down, and place it over Add content.
3. Click on Page.
4. Type the name of the page in the Title box.
5. Type or copy and paste your content in the Body box.
6. Scroll down to Provide a menu link and click in the box. A check mark will appear.
7. Ensure that the Parent item is your main menu topic, such as About Us.
8. Click Save at the bottom of the window.
Repeat this step for all desired secondary menu links (no more than ten).