How to learn backend development?

I have been trying to learn web development for a while but I can't seem to get anywhere with backend. I've learnt all languages(HTML CSS JS JQuery Ruby PHP) provided on codeacademy and have been using python for 2 years now. I also did a course on Databases on Stanford online.

But I don't know what should be the next step to build a backend for a website. Every search on google simply points to some basic suggestions - learn PHP,SQL or python, ruby, node.js etc. I know all of these except node But still don't know how all these languages are used to make a backend.

What should be the next step in learning backend development considering all the work I've done so far?