Jump into jQuery! (tm)
I've been spending a lot of time these days learning jQuery, the JavaScript library which comes bundled with Drupal 5.x. The good news is that it's really cool and powerful. It essentially allows you to forget about all those JavaScript things that fill you with dread: complex DOM iterations, browser inconsistencies, event capturing, etc. The bad news? There's a bit of a learning curve, especially if you've been avoding JavaScript because it fill you with dread.
If you know some JavaScript (and/or some programming fundamentals), start with Simon Willison's jQuery for Javascript Programmers. (And while you're at it, add Simon's site to your feed reader.) If you want something more practical -- a 'dive-into' tutorial -- there are plenty out there. Start with the tutorial page of the jQuery site. Also, if you're like me and prefer a book, there's Learning jQuery.
If you enjoy jQuery and would like to learn more about advanced javascript and javascript libraries, or if you'd simply like to crawl inside the mind of jQuery creator John Resig, I have a few recommendations: Resig's Pro JavaScript Techniques; Resig's blog; and a presentation he recently gave at Google, Building a JavaScript Library.
Finally, if you're really interested in jQuery, there's an all-day jQuery camp next month, right here in Boston.
p.s. - How could I forget? Don't forget to a) upgrade the jQuery in your Drupal install and b) bookmark the jQuery API browser.
p.p.s. - One more thing. Learn JavaScript before tasting the library kool-aid.