Here you can see how to add different variants of the same store product i.e. size, colour, material, sex.
Go to Store>Products>click on the desired product.
From the next page click on edit and choose the attributes you would like to show i.e. Size, Colour etc.
Once done, you will be able to click on each product variant (pencil icon), and add the attribute values of your choice:
i.e. if you had Small and Medium T-Shirts to sell, you would setup 2 product variants, one for each size.