The complete doxygen-generated API reference guide is the best place to find detailed information about the classes available. (If you want to download an offline copy of these API docs, they're available as a github repository).
A "getting started" guide is available here.
The "Getting Started with JUCE" book is available here!
If there's something you can't find or understand, try searching for it in the forum. If you still can't find it, please ask on the forum!