SCJP K&B is a really excellent book to start.
1. code, code and code
2. clear your doubts on this forum by putting your questions and telling which options you think are correct and why
they are correct.
3. Try solving the questions and doubts of fellow ranchers.
4. Do mock exams given in the SCJP links. The free ones like inquisition and devaka coorays
examlab are really
good. But go for them only when you think you are ready.
But the key is to understand the concepts which is only possible if you code and see the results for yourself.
It may vary from person to person but if you are new to
java then ideally considering the difficulty level of the exam not more than 2.5 months are needed to prepare and have a final crack at the exam.
If you are already working on java then 1 month of regular preparation should be sufficient.