Ankit is right ... you should check the FAQ ... but I can save you a little time ... this is what you should do ... its what I'm doing and many others Java ranchers will attest to this ...
Buy the K&B book - Head First Servlets and JSP (2nd edition)
Download the specs - Servlet and JSP
Buy the EnthuWare mock exam
Answer questions on JavaRanch
The K&B book is easily the best prep book for the exam. You'll need to occasionally see what the specs actually say, but the K&B book has almost everything you need. Some people like the Whizlabs mock exams ... but I personally prefer Enthuware ... also there are really good free ones too! There are several nice free ones if you dont want to pay ... but I still like Enthuware. Lastly, try to answer a few questions here! Looking up the correct answer to a question somebody has posted and then explaining it in writing will help you learn and remember! Other JavaRanchers will correct you if say something that's wrong and that will help you too!!