Read the JavaRanch FAQ, and I personally recommend Dan's Exams, and the examulator
Also, the "Bert and Kathy" book appears to be the best book out there for SCJP.
If you already know Java, it shouldn't take you too long to get up to speed. Give yourself enough time, and you'll know you're ready when you can revisit some of the mock exams and can answer most of the questions pretty easily without guessing.
Also, it helps to visit these forums often, read some of the problems posted, and maybe even answer a few of them. You learn a lot by trying to explain certain concepts to beginners
[ July 09, 2003: Message edited by: Brian Joseph ]