Take a course:
https://www.learnstreet.com/ – A highly interactive course that allows you to learn and play at the same time.
Learn on your own:
http://jsfiddle.net/ – The best online interactive testing and collaboration tool.
I’m not going to list w3schools (unlinked) – w3schools is a definite resource, but only as a reference once you understand the concepts and basic syntax.
Learn by example:
http://jsfiddle.net/BH8q9/ – Countdown
http://jsfiddle.net/8LV3p/ – Use Chrome / Firefox for this one!