how to prepare ?

first go throgh  complete syllabus of NDA. then on the basis of Syllabus and your knowledge of subject make a routine for study. follow that routine everyday sincerly. every make  some target and try to complete those target.
speacilly work on mathematician part. because if you are not from science background then it will be difficult for you to solve them. so give more time on mathematics.
after preparation try to solve some sample and previous years question paper. that will give you more confidence and you can know your ability too.