What should I learn after HTML, CSS, and JavaScript to build an online store?

For a real store, consider building the storefront yourself but using an established commerce service for products, stock, orders, and transactions. Shopify is one option with an API. That lets you practice the web development side without having to build every part of a store from scratch.

You'll still need to learn how the API works and how to connect your frontend to it. Check the service's documentation for what it handles and what remains your responsibility.

Keep the first version a practice project rather than a live store taking real money. Bugs or security problems can cost customers money, and there are consequences for the person running the site too.

When you're ready to explore payments, use an established provider such as Stripe instead of writing payment processing yourself. Follow its documentation carefully. Using a provider doesn't remove the need to secure the rest of the site.

There's more to the backend than storing a list of products. Think about permissions for customers, employees, and administrators, plus how products get created and updated. Search and filters are useful things to explore once the basics work.

Use mock data while learning and write automated tests for the basic create, read, update, and delete operations. That gives you a way to check the core behavior as the project grows.