If you Option-double-click on any text, the Organizer will search for that string.
If you Command-click on any class, function, or method, Xcode will show you the file where it is declared. (Try Command-clicking the word “NSMutableArray”.)
If there are several declarations that match, you’ll get a pop-up when you Command-click. Try Command-clicking addObject:.
From time to time, Xcode will call back to the mothership to see if there is any updated documentation. It will download the updates and re-index them for easy searching. You can trigger an immediate check for updates in the preferences panel for Xcode.
Apple hosts a collection of discussion mailing lists at http://lists.apple.com/, and several of them are devoted to developer topics. If you can’t find an answer in the docs, try searching the archives of the lists. (As a courtesy to the thousands of people on each list, please search the archives carefully before posting a question.)
There is one more resource that you should know about: stackoverflow.com. This website is a very popular place where programmers ask and answer questions. If you can think up a few keywords that describe the sort of problem you are having, you can probably find an answer there.