First of all, do NOT start by using a javascript library (prototype, jquery, etc). Learning a library isn't half as useful as learning the language.

So, get a copy of O'Reily Definitive guide, Start working through that. Watch the Douglas Crockford lectures from Yahoo.

Install firebug and start messing around.

"Hrm I've read about the arguments.callee property. Lets play with that for a few minutes and see how it works" etc etc

The main thing is just to explore, get down to the nuts and bolts, see how it all really works. Try things out, and see if they work or not.