What knowledge I need to start creating extensions?

Hello there guys, I have a little knowledge about programming and I was wondering what I need to know to start making a extension. I already did some chat bots and another things not web related.

I know it’s a really beginner question, but if I could get any information about what to study would be great.

look, the simplest way I see to create something of basic level, is using html, css and javascript, if you give more details of what I would like to create, we could direct you better :grin: