To become a full stack developer, you have to be able to work with both the backend and the frontend. That sounds like quite a challenging task, but if you focus on the right things and don't try to master everything, it is possible. Nowadays, the technology stack available is huge and you might often wonder what you should learn next. There are multiple factors that might give you some hints about where to go next. One way is to browse job opportunities and see which technologies companies are looking for.
There are multiple ways, and no right path, to start learning a new technology. The usage of programming web courses is a really popular starting point and that gives you basic knowledge to start the learning process. The process is never-ending because technologies are developing and changing all the time.
The following technologies are necessary to understand if you want to become a full stack developer. This is not the complete list, but it is a good starting point...