Hey,

There are seven objective-type test papers in IIT JAM: Chemistry (CY), Physics (PH), Mathematics (MA), Geology (GG), Biological Sciences (BL), Mathematical Statistics (MS), Biotechnology (BT). 30 questions having 10 questions of 1 mark each and 20 questions of two marks each. Each question will have four choices out of which only one choice will be a correct one.
It will be beneficial for you to begin your exam preparation during your graduation. Most of the questions are asked from what you study during your graduation years, therefore, you must be very thorough with the degree course.It is indeed important for you to clear your concepts and have a proper understanding of each topic/chapter. Also, to keep your preparation on track, you must also make notes while studying for the exam.
Depending on the course that you are opting for, getting acquainted with the syllabus is the first and foremost thing to do.The syllabus of IIT JAM is announced by the organizing institute or respective institutes well in advance. Therefore, be sure to prepare each and every topic that has been mentioned in the IIT JAM Syllabus.
The more you solve the previous year question papers, the better understanding you get about attempting the final exam.
There are various IIT JAM books by different authors. Referring to these books while preparing for the exam will help you extensively.

Hope this helps.