Creator of OMFramework (2011)—an OpenCart extension that helped store owners have a mobile site without costly theme redesign. E-commerce Usability and Conversion Rate consultant since 2016.
Start simple with these technologies and build on that stack over time. If you start making websites with Bootstrap, React or something else, you'll quickly be overwhelmed and you'll miss on essential basic understanding.
The first step for you now would be go explore CodePen, make an account and use that as a playground. It greatly reduces your moving parts, as you won't have to think about setting up a development environment.
As to what to build and get your hands dirty with—find simple CodePen examples that you like, copy them and play with their code. Try changing things here and there and see how this plays out.