Best way to learn it is to give yourself a bite sized project.
Simplify things by choosing 1 environment to begin with (ie, if the JS
will run in a web browser environment, then choose one browser to begin
with - note IE will teach you bad habbits).
Learn the core language first before getting mixed up in the document
object model (DOM). DOM brings with it a host of separate issues.
You will do well to begin your journey by reading this guide
http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Guide
Read all of it. Don't skim, and don't miss anything. You miss a detail
in JS, and you pay for it later in debug time.
Clear your mind of prejudices and expectations about the language. You
only find out how powerful it is if you are receptive to it's unique
approach to scoping, typing, and object architecture.
good luck.
ps. Take the time early on to familiarise yourself with Venkman. You
won't regret it. I can't live without it now. It's a great tool for
disecting JS even as you browse other people's page in your web
travells.
For firefox 1.5, you can get venkman here:
http://getahead.ltd.uk/ajax/venkman
Official home:
http://hacksrus.com/~ginda/venkman/
Don't try to learn Venkman thoroughly initially. just learn your way
arround it, how to set break points and watch expressions.
