If you are brand new to Javascript, a good place to get your feet wet is the Javascript tutorials at w3cschools.com. Webmonkey.com also has a series of Javascript lessons by David Thau for beginners and more advanced Javascripters.

If you are somewhat familiar with code and/or Javascript, Mozilla Developer Center has a very good overview of the Javascript language. They call it an "Re-introduction to Javascript" because it has evolved quite a bit over the years and as Douglas Crockford say is "the world's most misunderstood programming language". If you learned the basics of Javascript a while back, this is a good place to get re-acquainted with it.

Douglas Crockford, architect at Yahoo, gave some beginning and advanced Javascript lectures linked here for those who rather watch than read.

Code Conventions

Start learning good coding style and conventions. Douglas Crockford has a list of coding convention that we recommend that you follow. And also run your Javascript through the Javascript validator JSLint.com.

DOM

As your gain sophistication in your Javascript, you may have a need to traverse the DOM tree and dynamically create elements on the DOM, when "DOM" stands for the "document object model". The video lecture about the Theory of the DOM will then be helpful in your understanding of the DOM.

If you want a book on Javascript, Crockford recommends (in the first video) the book JavaScript: The Definitive Guide.

Debugging Tools

There are debugging tools for Javascript such as Firebug for Firefox, Microsoft Script Debugger for IE, and Venkman for Mozilla.

Javascript Book

JavaScript: The Definitive Guide Preview the book » This book starts by covering all the fundamentals such as data types, variables, operators, functions, and arrays. Then it goes into Objects and object-oriented Javascript. Advanced topics includes scripting cookies, DOM, and event handling.