I think the answer lies somewhere in your question. "You never used it" so you end up forgetting it. You just need to practice everyday. There are several books you could read to begin with and know the fundamentals of JavaScript, e.g 1) JavaScript: The good parts 2) JavaScript Ninja etc. Also websites such as codewars.com offers a bunch of programming task to get you hands a little dirty (you can try to solve one task daily).

How can I learn a new language fast and what are the most important things i should look out for while learning?

I do not know of any specific project to recommend to you but you could try doing some task at codewars.com or any related sites . Try as much as possible to learn and apply new JavaScript concept as you go on. You would begin to understand the concepts better and become a pro in no time.