Your first stop has to be the official BlackBerry developers site at:
http://www.blackberry.com/developers
All the SDKs, lots of tutorials, docs, etc
The official forums are also useful:
http://supportforums.blackberry.com/t5/BlackBerry-Development/ct-p/BlackBerryDevelopment
And I'd recommend a good book, there are 3 for beginners that come to mind.
BlackBerry Development Fundamentals by John Wargo (good overview of the platform and different approaches to BB development):
http://www.amazon.com/BlackBerry-Development-Fundamentals-John-Wargo/dp/0321647424/ref=dp_cp_ob_b_title_1
BlackBerry Java Application Development by Bill Foust (just came out, I haven't read it yet but it's gotten some positive feedback):
http://www.amazon.com/BlackBerry-Java-Application-Development-Beginners/dp/1849690200/ref=sr_1_5?s=books&ie=UTF8&qid=1281984330&sr=1-5
and last but not least Beginning BlackBerry Development by me! (Java application development, seems to have gotten a good reception too):
http://www.amazon.com/Beginning-BlackBerry-Development-Anthony-Rizk/dp/1430272252/ref=sr_1_1?s=books&ie=UTF8&qid=1281984428&sr=1-1