The 101 series is a prime place to start. Themes check out the media area. OSes look at the sticky at the top of the 8900 forum or go to beta OSes (make sure to read before doing - SERIOUSLY)
Check out the 101 lectures, I have a link to them in my signature. You can also find the link at any time using the "Blackberry Help" menu link at the top of every page.
Welcome to CB! You will get used to your new Blackberry soon. Got mine for a good week now and would not want to part with it anymore. My iPhone 3G is going out the door!
Posted from my CrackBerry at wapforums.crackberry.com