Hi,
1. At first you have to know your strength and weaknesses.
2.Follow NCERT books of classes IX and X for NTSE preparation. For some concepts in biology, refer NCERT Class XI Biology textbook (only portions that are relevant to IX and X class curriculum). Its imperative to thoroughly study the Social Studies NCERT book for NTSE.
3.The MAT section tests your mental ability. Hence, you must give sufficient time for preparing for this part.
4.You must solve a lot of sample papers and mock tests at least thrice a week. There are a lot of NTSE.
5.For your reference book please visit this link
https://www.toppr.com/bytes/ntse-books/

All the best.