Sorry friend,
After 12th,you can not apply for UPSC exam.
Age limit for UPSC exam :
>>Age is between 21-30 years.
ST/SC gets 5 years age relaxation.
OBC gets 3 years age relaxation.
Eligibility criteria for UPSC exam :
>>Graduation pass from any recognized university.
Condition :
>>Candidate must be a citizen of India.
Selection process :
>>Preliminary exam
>>Main exam
>>Interview
Attempt :
>>4 for general
>>7 for OBC
>>No limit for ST/SC
You can check here for details............
http://www.upsc.gov.in
Now,you can apply for NDA exam.
NDA exam conduced by UPSC.
Eligibility criteria for NDA exam :
>>12th pass with math,physic,chemistry pass from any recognized board(for Navy/Air force).
>>12th pass with any stream from any recognized board(for Army).
Age :
>>Age is between 16.5-19 years.
Condition :
>>Candidate must be a citizen of India.
>>Candidate must be a male candidate.
>>Candidate must be an unmarried candidate.
Selection process :
>>Writing exam
>>SSB Interview
>>Medical test
Syllabus :
Physical ability :
You can check here for details............
http://www.upsc.gov.in
Good luck !