There's a link to "certification" on Sun's
Java homepage (which you will know where to find if you have done anything in Java).
That link will lead to a page (after a few clicks) detailing the content of the exam.
That page will tell you what you need to know about, you can even book the exam online right there so it's good to know where to find it
.
For the rest, searching through this forum and the rest of this site will yield you a wealth of information.