Firstly you need to know what is the syllabus for NDA examination. Questions will be asked from 10+2 level only. It consists two papers. Paper-I covers Mathematics. For this you need to study algebra, matrices and determinants, trigonometry, geometry, differential calculus and equations, integral calculus, vector algebra, statistics and probablity. Paper-II is general ability test where it has two parts. In part A- there is English and in part B- general Knowledge(physics, chemistry, general science, history, geography and current affairs.Finally after cracking NDA there is a final round that is intelligence and personality test.

So for aspirants belongs from humanities background, you have to study physics, chemistry and also mathematics. You can read ncert's 10+2 PCM to make a strong base. Then further read abc physics and chemistry books and for mathematics first solve ncert then solve and practise RD sharma Maths. Finally after doing all this tasks solve previous year question papers which will let you know what kind of questions from PCM comes in exam.You can persiue coaching classes as well. It's not easy to manage all by yourself. It's seems hard but not impossible. All the best.

1. Check all eligibility criteria from upsc website.
2. Read official notification of upsc for syllabus.
3. Now revise math, physics and chemistry section from your 11th and 12th class books according to the syllabus (after 12th)
4. Buy previous year solved papers.
5. Give mock test and practice for English from mocks.
6.Read English newspaper to improve vocabulary and language command and current affairs.
7. Read lucent book for general knowledge.
8. Practice...
All the best