I did the courses on ruby, and now I'd like to create a web app, but I have no idea what I have to do now, what kind of program do I need etc. ...
Well, ruby sounds like a rails, and the ruby on rails on framework as well.
Yes, i know this is difficult, but you need to this yourself to understand it.
Oh, one more thing, many websites backends (which rails is) run on linux (if the platform is not a problem), i don't know what OS you are running, but give it a thought. I personally think development for this kind of thing (rails, ruby) goes much better on linux (great shell, tools, guides and so on)