How to create an interface for a website's manager to upload images?

I have just started studying web-dev and I am familiar with the basic html, css and js concepts.

I am working on a gallery website, which is made of bootsrap html templates I found online and adjusted. I want to create an interface for who will eventually run the website, to be able to manage it - open a new gallery, upload a new image which will be contained in a specific gallery, deleting, ext.

What is the best way to do this? what should I learn?

Thank you!