hi Keerthana,
As people have suggested here, you can use Head First Servlets & JSP to begin. Like most other books of this series, they are fun to read, and you learn the stuff pretty well without mugging up or getting bored. I just finished that book, and it was a good experience.
For certifications, you can use
SCJP Exam Guide by Kathy Sierra and Bert Bates. Its tailor made for the exam, because the authors are part of the team that designs the exam.