There are a number of resources to learn jQuery (which is completely worth it IMHO). Start here http://docs.jquery.com/Main_Page to read the jQuery documentation. This is a great site for seeing visually what it has to offer:
http://visualjquery.com/1.1.2.html. Manning publications also has a great book which is highly recommended called jQuery in Action. As far as JavaScript libraries are concerned, this one and Prototype are probably the most popular if you're looking to compare jQuery to something else.
