If you are starting on Javascript, I would recommend you going to this website.
However, if you already know bit of javascript, then it all depends on practice. This is the only way i know of that can make you a good programmer. And this apply to just about anything and any programming language.
Try to come up with your ideas and materialize them using Javascript.
How about building your own lightbox?
How about creating your own table filtering/sorting?
And how about all the cool stuff you might have in your mind?
Thanks :)