JavaScript

Javascript (often abbreviated as JS) is a nifty scripting language that runs in the browser. If you don't know what that means, just know that Javascript is what makes, say the Gmail email addres completion or the Facebook search work.

You will probably use javascript to build most of the functionality of the interface of your applications.

Note. Javascript has nothing to do what so ever with Java.

First Steps
For starters, do yourself a favou and make a CodeAcademy account. Then follow the JS Track.

Moving Forwards
So you've taken the first steps above and you're starting to get JavaScript. Do not move on until you have a basic understanding of how the language works. Seriously, do not move forwards until you understand what the JS code does, don't start mindlessly pasting JS snippets from the web just to do funky things;

If you have time, "Javascript is Sexy" has a great 8 week itinerary to learn Javascript on your own. They recommend 2 books, "Professional Javascript for Web Developers" is the one to get, it's amazingly deep & clear at the same time.

When you're confident enough, however, you can move on to these:
 * jQuery
 * coffeescript
 * ember.js