How can I add a main category of products?

You can add a main section of your store products through the following steps:
Go from the control panel to the products then categories and then click on the icon Create a new category ... Next image for clarification

Then enter the page category settings through which the name of the section is written in the languages listed on the control panel of your store and  can be add picture of the section and write description for it ...

Note: In order for this category to be primary, you must select (None) the Parent tap as shown in the following image:

There is an icon called preview, which through click on it to see the category after all the setting done before saving ... Next image for clarification

After all the settings are done pressed icon save