It depends on your interest & capability. After 12th you can apply for Staff Selection Commission (SSC), every year hosts exam for Higher Secondary Level (10+2) for the posts of:

• Data Entry Operators (DEO)
• Lower Division Clerk (LDC)
• Stenographer (Grade ‘C’ and Grade ‘D’)
• Non-Technical Staff

And as you have mentioned you are also interested in diffence then Apart from it, UPSC also hosts exams twice a year for the recruitment of staff in Indian Army, Indian Navy, and in NDA. So you can also apply for the exams.

• CDS Exam
• NDA Exam

Or

• SCRA (Special Class Railway Apprentices) Exam held by Indian Railway
• Recruitment for Soldiers / Mahila Constable (General Duty)
• Junior Commissioned Officer (Catering)
• TES- Technical entry scheme, only for male candidates, who passed 10+2 with PCM stream.

To apply for these exams you need to be updated yourself.

Good Luck!

Choosing a field after class 12th totally depends on your interest. If you are interested in government job you can go for below entrance exams-

1- Combined Preliminary And Mains

2- NDA

3- Technical Entry Scheme

4- Clerical Exam

5- SSC

6- Indian Navy Recruitment

7- Marine Engineering and Research Institute (Merchant Navy)

8- Navy

9- Bank

10- Special Class Railway

11- Intelligence Bureau

12- UPSC LDC

Good luck!!