Hello Gaurav,

Firstly I would like to inform you that "there is no reservation in the defence forces". 

All the candidates are selected after on merit basis. There are three stages in the selection procedure of NDA:

1- Written test

2- SSB interview

3- Medical examination

After passing all the three statges you will be selected.

NDA conducts a national level examination is held twice every year. More than 3 lakh candidates appear in the written  examination. The basic eligibility criteria that is required for a candidate to appear in the examination is:

I. Nationality / Citizenship:

A candidate must be either -

(i) a Citizen of India or

(ii) a subject of Nepal or

(iii) a subject of Bhutan or

(iv) a Tibetan Refugee who came over to India before 1st January 1962 with the intention of permanently settling in India or

(v) a person of Indian origin who has migrated from Pakistan, Burma, Sri Lanka, East African countries of Kenya, Uganda, the United Republic of Tanzania (formerly Tanganyika and Zanzibar), Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.

II Age:

The candidate's age should be between 16.5 years and 19.5 years.

III Educational Qualification:

The candidate must have passed or is appearing in the 12th examination.

For naval and air forces, the candidate must have Physics and Mathematics at 12th level.

IV The candidate should be an unmarried male.

V The candidate should be mentally and physically fit.

The minimum marks a candidate needs to score to go to the next level is called cut-off marks

Now the cut-off for the written exam vary from exam to exam depending upon the number of candidates appearing and difficulty of the question paper. Cut-off marks are given out of 900 and there is also sectional cut-off.

The last year cut-off for the first stage was 338 with 25% sectional cut-off. And after the SSB interview the final cut-off was 705 out of 1800.

So after considering all the previous years cut-off marks we can say that a candidate must score at least 40% marks to get selected to the next level. 

So if a candidate scores more than 360 in the written exam he has a very good chance of getting selected to the next level and he must start preparing for the SSB interview.

The exam is held offline and paper of NDA is divided into two sections:

1- Mathematics ; which consists of 300 marks

2- General Ability ; which consists of 600 marks

You can refer to NCERT books and Arihant for Mathematics section and books by Pathfinder and TMH for General Ability section. 

All the best