You can make items be available to your users by simple using class names in your html. For any Item you want to be available to be added to the cart, you make a container with a class name of evoShop_shelfItem. Then add classes to tags inside of that container that have the general form item_[name of field] and evoShop will use the value or innerHTML of that tag for the cart. For example, if you wanted to sell a T-shirt with 3 different sizes, you can do this:
Notice here that you can use a select to change options for the item when you add it to the cart. You can also use a text input to change the quantity (or any other field!). These classes will work with any tag, so feel free to use what works best for you. Finally, notice that a tag with the class item_add will have an event listener on its click. So when the contents of that tag are clicked, an item will be added to the cart with the values of each of the tags in the container with the item_something class.