Hello @Canne_Pham ,
You can prefer documents of shopify which is well described in details.
Here is the link it will help you.
For your question on adding variants an existing product on Shopify, here is our article on the topic of how to add variants to an existing product on Shopify https://avada.io/shopify/docs/how-add-variants-to-existing-product-shopify.html
P/s: If you have any trouble while managing your Shopify store, this site about Shopify may help you https://avada.io/shopify/docs/
Hope it helps!