Hello there!!

Its better you join a college and start your undergraduate studies.And you can also appear for the NDA exam in September.If you clear the exam and SSB and are also medically fit then, you can join the NDA and leave your college.

Your college will act as a backup in case you do not qualify at any stage of the selection process.

Hello Nani, 

Look first thing first, when you join any Defence service through NDA you cannot pursue any engineering branch. So far National defense academy does not provide any engineering course. They only provide BSC courses. 

However, to join defense service with engineering course, you can apply through technical entry after 12th. For that you need minimum 80% in 12th. So after SSB, you will go OTA. 

Now as you have mentioned you also want to engineer. So you can pursue aeronautical engineering first and then apply for exams like AFCAT, which require graduation in aeronautical engineering or mechanical engineering. Other than this there are other options for engineering entry after graduation such as UES, SSC(Tech) army, TGC army and some entry in Navy. 

Hope you find in this information useful and have a great future ahead.