Well I wont tell you more about this exam, but would tell you what to do and what not to do for this exam.
If you are an undergraduate student, and want to go masters, don't think too much about it until you are in third year second sem. But I would suggest you to do some sort of research/ project/internship till third year and write your GRE exam in the third year holidays or at the very start of 4th sem.

REASON: Its not only GRE and Good Acads which would land you in a good univ, but some amount of research work atleast to get into a good univ below 50 ranked and also for assistant ship. And also you would have lot to write about in SOP( statement of purpose) about your research. This would show how much practical you are and keen to do research.

Booking a GRE Exam: Since GRE is a computer adaptive exam, where the test gets harder as you start answering correctly. But here's some thing I would like to share. GRE database has a set of question for the first 15 days and another database for the next 15 days. so if you book your exam during the last days of the 15 days of the month or at the end of month, you would be litlle lucky to get some repeated questions. There are people who share the questions after they write their exam so who know you can get lucky some times.

But dont take any chance and prepare. HOW TO PREPARE FOR GRE.