How to start backend programming [on hold]

let's say I want to write an App like YikYak or Twitter. How would I implement it's server side backend? How would an "architecture" of an implementation for this look like? How would I manage multiple users and processing of their interactions? How would I scale something like this? What frameworks/languages should I use? I have no idea where to start.

I already know Java, C and some Python. Even if I find some frameworks for server programming, I'm asking myself what to do with these, even though I have got some experience with programming.

Thanks a lot, J.