A good starting point for learning new framework is to buy a book about. The problem with books is that they get quite fast out-dated for fast developing framework and that is also true for the Apache Wicket project. But quite recently a new e-book have been published for Apache Wicket 6 and the best thing about it, it is free - http://code.google.com/p/wicket-guide/.
You can get a complete list of all Apache Wicket books here http://wicket.apache.org/learn/books/.
And if you want another book I would recommend Wicket in Action.
Another good starting point is to run the components examples (http://www.wicket-library.com/wicket-examples/index.html) on the Apache Wicket page and read the Reference Wiki pages - https://cwiki.apache.org/WICKET/reference-library.html.
The next step if you have not already done that is to start writing codes. When doing so it is good to knew that there is a very good mailing list group for Apache Wicket, which I recommend to search in, if you get stuck - http://wicket.apache.org/help/.