A full blown e-commerce is complicated. My first suggestion would be to look at existing solutions and start from there. Magento is one of the most widely used, written in PHP and Open Source. It's proven and works. It's also heavy on resources and complicated.
If you haven't build a full e-commerce site I would also suggest to look at the APIs of Shopify and Ecwid so you get a general idea of what you need to build for success. And maybe use them as a backend.
If you still want to build one (to learn for example), keep in mind that Putting a Catalog online (Products, Categories and Stock) it's easy, but you'll need quite a few more things for e-commerce:
Tonnes of reports and everything with security in mind. You also need to keep in mind which country are you selling in, because depending on that things might change. And keep an eye in fees and read about charge-backs.
Finally, if after reading all this you still want to pursue this endeavor. I'd suggest to create an account on Shopify or Ecwid and use them as backend, both start free. I think in Ecwid you don't get charged until you sell something.
Then start building your own solution and replace bits and pieces as you get more experience.