As you said you are in class 12th. First complete your 12th successfully with best score. Once you complete your 12th, give NEET examination see that you are fully prepared for examination.
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Now to become army doctor, you should join in AFMC which stands armed force medical college. The college accepts NEET examination for admission.
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From here you can become army officer or army doctor or any post in army.
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Or else once you complete your MBBS from any random College, army department announce jobs on there websites can also apply  via that.