I think the first step is understanding that there is JavaScript (the language), then there is the browser's Document Object Model (DOM) and "HTML5" APIs. The JavaScript language is used in more places than just browsers (e.g. Rhino and Node.js) and doesn't change very rapidly. The DOM and APIs vary more across browsers and get new features with just about every browser release.
I'd recommend learning the language first, then the basics of the DOM and subscribing to the main browser's blogs to keep up with their changes.
If you just want a site to learn JavaScript, I'd highly recommend https://developer.mozilla.org/en/JavaScript (as other people have too). Mozilla's docs are top-notch. They're a wiki so sign up and help make them even better.
I'd recommend learning the language first, then the basics of the DOM and subscribing to the main browser's blogs to keep up with their changes.
If you just want a site to learn JavaScript, I'd highly recommend https://developer.mozilla.org/en/JavaScript (as other people have too). Mozilla's docs are top-notch. They're a wiki so sign up and help make them even better.