Hello,

Yes, you should start preparing from 11th. In NDA written examination both 11th and 12th knowledge matters. First of all you need to aware of NDA syllabus.

Practice all the math topics from textbooks. Also follow other reference books as well. in the meantime you can also start preparing for general knowledge, it will be much helpful. Understand the concepts and build your base strong.

So, overall, study the syllabus of 11th seriously because you won't get much time to study 11th concepts in 12th. If you can grasp the 11th concepts while studying in 11th then it will be very helpful in 12th.

You can also check the link below to get to know about the best books to prepare for NDA for your reference

https://competition.careers360.com/articles/best-books-prepare-for-nda

Hope it helps!

Good luck!