How can I go into navy after completing my 12th with science (biology)?

Yes. Aadhaar is needed for registration, for face authentication on NSP, and for payment. The bank account must be seeded with Aadhaar for the money to arrive.

Central schemes are filed on scholarships.gov.in after One Time Registration. State schemes are filed on the state portal, though twelve states and union territories route their applications through NSP.

All four central post matric schemes use a family income limit of INR 2.5 Lacs per year. State schemes set their own limits, so check the notification on your state portal.

Yes. Class 11 and Class 12 come under post matric schemes. Both central schemes on NSP and state schemes accept applications from students studying in these classes.

There is no single best one. SC students should apply for the post matric SC scheme, ST students for the ST scheme, and OBC, EBC or DNT students for PM-YASASVI, along with their state scheme.