Hello Shah,
Welcome to JavaRanch. It is a great place for all level of
Java learner. First take
test at roundup section to see where you stand...where basic concepts are concernd.
You should get above 95%. Don't get dissapoint on lower score. Keep on practicing.
Also as Bill mentioned take part in the daily discussion. You can learn a lot while reading old/current post. But before looking for the reply of any post first you try to solve and figure out what could be the possible answer for the post. Then you check it with the reply submitted by various like minded people. Some are expert and most of them are like you and me trying to learn the things for making ourself good enough for the exam.
In short practice, practice, practice and hard work and you're contribution to javaranc in reading, posting and replying to the post will definately make you an expert in core Java. "One learn more by Teaching Other".
Regards,
Raj.