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The easiest way to do this is with the Online Store. It allows multiple levels of categories and is easy to maintain.
I see from your posting history that you do probably have an Online Store so to do the sub categories:navigate to
Control Panel > System Settings > Catalogue > Categories and there you will see the categories management including the ability to add a new sub-category to an existing category. This can be drilled further into a lower level of sub-category.