You can either add products manually one by one in Sitoo Backoffice, or import a larger amount of products at the same time with product import.
To add products manually, log in to Backoffice > Products. Then tap the add-on icon to manually add a product. See this page for more information on how all product fields work.
If you want to make a product import, use the import button found in the bottom right corner of the product overview. Go to this page for more information on how the import function works.