about USER account

I want to create a user on hikashop, who will be able to create categories only (nothing else). how can i do that?

You will have to add him to a specific user group via the joomla user group manager. Then, allow him access to your back end and limit his access to HikaShop with that extension:
extensions.joomla.org/extensions/access-...access-control/17951

Then, in the "access level" tab (business edition) of the configuration, you can select which feature each group can see/use in HikaShop.