I would like to learn backend, what is the path that I could follow to learn this approach of web development?
Actually I know nothing, which are the topics to become to back-end developer?
I don't think you should think of it as a set of topics. Here's the thing: if you want to be able to build a backend, the only way to learn how to do it is by building a backend. So go ahead and take up a small project and learn everything along the way while you do it. Another thing, I don't know what your experience in software development is, but it doesn't make sense to know just one of backend or frontend. You could specialise in expertise once you have an idea of both, but one doesn't make sense without at least a basic understanding of the other. For example, understanding HTTP and the way the browser requests and renders a page and makes AJAX calls is vital to developing both frontend and backend effectively. Some ideas for projects:
Hope that helped, cheers :)
I'd suggest starting out with NodeJS, since:
There are some awesome lessons on FreeCodeCamp.
Brandon
Frontend Developer
Nishant's example of a To-Do app is a good one. There is a website dedicated to building the same To-Do app using various technologies.
todomvc.com
After you pick something to develop I recommend you build it again using different technologies. Each time you build it differently you will learn more about what you're building and why.
The first backend project I built used Meteor. That taught me a bit about databases, subscriptions, and sessions. Next I rebuilt it using Polymer. That made me better understand routing, templates, bower, and components. After that I built it using React. That helped me understand states, properties, and rendering the virtual DOM. (Note there is no correct order to learn these things. I suggest you pick the one that seems easiest to understand and start there).
All of those used slightly different design patterns for how data can flow. What can modify data, when, how, etc.
Each time you build a new version of the program you can also experiment with different data stores. MySQL, Mongo, flatfile, APIs, etc.