If you are interested to join Indian Army & passed or appearing class 12th then you need to appear for NDA exam conducted by UPSC . As per NDA Eligibility Criteria you must be between 16.5 and 19.5 years of age and have passed 10+2/HSC.

Physical standard required for NDA - The candidate must be in good physical and mental health and free from any disease/disability which is likely to interfere with the efficient performance of Military duties. The minimum acceptable height is 157 cm for Army, Navy and Naval Academy while 162.5 cm for Air Force. For Gorkhas and individuals belonging to hills of North-Eastern the minimum acceptable heights will be 5 cm less.

For further information about you can go through with given below link.

https://competition.careers360.com/articles/nda-eligibility-criteria

Here are some of the preparation tips you can follow

Everyday 2-3 hours study is necessary on regular basis. Categorize the course as per your comfort and start preparing.

Practice previous year question paper as much as possible.

Reading monthly magazine of 2-3 months prior to the date of exam will be helpful.

Time management is a key here, see how many questions you are able to answer in minimum amount of time.

When practicing previous papers think once before guessing on an option, since negative marking is implied here so every wrong answer will cost you extra 1 mark.

Learn shortcuts, avoid length calculation on paper do it in your mind.

Learn tricks, in mathematics on few questions answer can be brought up by looking at the options. Instead of solving the whole questions take the given options and solve the question around those options.

Watch good informative programs on TV, read extensively newspaper, magazines, journal etc.

Study in group, it help you in sharing the ideas and grasping the concepts quick.

Good Luck!