Welcome to the YUI library! This tutorial contains everything you need to get up and running quickly with YUI. If you've never used YUI before, but you perhaps have a little experience with plain JavaScript or JavaScript frameworks, read onward.
Note that if you have experience with jQuery, it is a great idea to read or at least skim through the JS Rosetta Stone, which demonstrates how common jQuery and YUI idioms map to each other. The good news is that in the areas where YUI and jQuery overlap, you'll find that it is not too difficult to translate back and forth between the two.
The easiest way to get started with YUI is to use SimpleYUI, a convenient package for working with DOM nodes, events, UI effects, and AJAX.