Hi,

First step is to enroll yourself in a good coaching or a course. You can enroll in CATKing's course as they provide quality content worth preparing from. They provide notes and mock tests which help you prepare yourself for the examination. The mentors there are those who have aced their NMAT exam and provide great strategies. You can also refer to books such as CAT Quantum, Nishit Sinha and Arun Sharma. These books are great for preparing quantitative aptitude in NMAT.

I hope this helps,

All the best.