I want to make my career in defence sector as my family has many army officers. With which subject should I pursue B.Sc?

Kindly suggest some colleges for the same? I have just passed my HS with 70% only under state board (Assam) SEBA. I am not that strong financially but I do keep an aim to serve the country as an army officer. What should I do now? I have heard that Dehradun is the best place for such studies.