Welcome to Crackberry Nation!! I use the site to help me find more ways to use my 'Berry and fix it when I mess things up, lol. I've found it to be a great resource for themes, wallpapers, ringtones and applications as well!
If you're completely new to BB's you should check out the Blackberry 101 series out! It is a great well of knowledge
Check out the BlackBerry 101 How-To Lecture Series ! This will help you with the basics such as downloading apps, themes, wallpaper, ringtones. It's also a good idea to browse the forums and read as much as possible. Many questions you have are answered there already!