Hey Vanna, 

If you go to sales>product and service> new>  you can choose what it is for example if it is stock, click stock fill the information in, the name of what this sub is and then the category this stock will fall under. so for example, if it is hair products the category might be hair products and the name of this new stock is hairspray this hairspray will be a sub under the category of hair products. 

If you have any quires let us know, we will be happy to assist you.