When you create products with variations such as a T-Shirt with multiple sizes, you need to create attributes for that product type. Each attribute will have 1 or more values. For example, a T-Shirt attribute could have values of Small, Medium, Large, Extra Large, etc.

As with my other introductions to WooCommerce, I will be teaching specific ways of managing WooCommerce. I recommend you to search WooCommerce Docs linked below for additional information.