Government & Defence Entrance Exam

Hi
To join Indian Army, navy or airforce as an engineer, go through the following links.
1. http://joinindianarmy.nic.in/
2. http://nausena-bharti.nic.in/forthcomingOfficer.php
3. http://www.indgovtjobs.in/2014/08/IAF-Career-Vacancy.html?m=1
Hope this suffice

History, Economics and Politics are the main subjects which are useful to clear IPS.

Hi
Check your rank from rank predictor at shiksha and look for the cutoffs for last year to see where u can apply for engg. For NDA, you may see the details here for eligibility.
http://www.exam2016.in/upsc-nda-i-application-form-2016-17-upsc-gov-in/

NDA required 60%in your 12th and 6/6 vision and a tough exam so I will not suggest to repeat your class 12th instead you go for b.tech this year. If you wish to drop than you can repeat your 12th but it slightly effect your preparation for b.tech

SSC CHSL is a competitive exam based on the notion of testing candidate's sharpness and accuracy. If your basics of the subjects of the syllabus of SSC-CHSL are good, you can crack this exam by doing lots of practice.
I am recommending you some books which you can refer
1. For English : S.P. Bakshi
2. For aptitude: R.S. Agarwal
3. verbal & non verbal reasoning : Kiran publication
4. Gk : Lucent is best
SSC CHSL Application form will be released from 2 July . Click here to apply for SSC CHSL exam 2016
https://scoop.eduncle.com/apply-online-ssc-chsl-ldc-application-form