technically you "study" for the NCLEX while you are in your nursing program. Then most students start a review, using various programs and books, shortly following graduation or even shortly prior to graduation.
This review is generally question banks that offer tips on answering the different type of questions, help to review information learned and the opportunity to identify and brush up on areas of weakness, and build testing stamina (tip: prepare to answer the max number of questions so you don't risk testing fatigue regardless of the number of questions you get.)
Depending on where you are in your nursing program, you may also find these sources helpful during your course testing since programs design exams in "NCLEX style."
Best of luck.
There is no grade for the NCLEX.... it is PASS or FAIL.