arul,
Check out the
SCJP forum on this site, there are a lot of great resources there. The short answer to your question is that you prepare by studying the objectives for the exam. You will have to know a few core clasess (things like
String, Math etc.) but you certainly don't have to memorize tons of them.
Check out the cert forum and focus on why things work they way they do in OO and you'll do well.
Best of luck!