Making a simple Duolingo-like Web Application. Where to start?

I would like to build a site similar in functionality to the Duolingo web app. I.e. it must have challenges similar to Duolingo's, linked discussion threads, user profiles, forum and achievements system. I would like to keep it simple, robust, lightweight, neat and fast.

The thing is I am not any kind of developer and I have no goal to become one. I am just taking all this as a hobby. But if needed I never had any problems to go in depth and get technical. For example, I played with a few programming languages in the past.

Currently I already scratched the general theory a bit in addition to some basics of HTML, CSS and JavaScript. But the problem is I am not content with that kind of blind learning anymore. To say it again I have no goal to make a full-fledged developer out of myself.

So I would like to know what exactly I need to get familiar with and exactly how deep to be able to work on my project as soon as possible. Also I would really appreciate any general thoughts and advices on the subject (mind you, being a noob I had a problem installing IDE a while ago as I simply failed to remember or google out what this type of software is called).