If you have only basic knowledge of
Java and you're going to do the
SCJP in a few days without properly preparing for it, then it's going to be very hard to be successful.
How long you need to prepare for it depends on the person and on how much you already know about Java. Most people take a few months, or at least a number of weeks to prepare for it. It's not something that you can really properly prepare for in just a few days.
See the
SCJP FAQ for more information. Many people use the excellent
SCJP Study Guide by Katherine Sierra and Bert Bates (the book is commonly called "K&B").