Best Option at 10 2 Level is NDA & NA Exam - National Defence Academy & Naval Academy is being Conducted by UPSC.
You Should Concentrate on 12th Level Math which consist of 200 Number ( paper-1).
Also concentrate on English , General Studies, Physics & Chemistry - 200 number ( paper - 2 ).
After that Interview ( which is toughest to crack).
You can appear in exam along with 12th Class as appearing in qualifying exam.
The exam is conducted twice a year ( generally April & August).
I also cracked the written exam in August'11 but didn't appeared in interview.