If your age is under 19.5years and above 16.5years and are unmarried male candidate then uou can join Indian Armyby appearing for NDA exams. 

You can alternatively go for Mechanical engineering and go for Campus/Offcampus Recruitment for Indian Army for senior position where age criteria is relaxed to 21years.

You xan simultaneously check eligible criteria for all different posts. 

http://joinindianarmy.nic.in/Alpha/ApplicantEligibility.htm