First of all, you should study hard your college syllabus. NDA mainly compromises of your 10+2 subjects. Apart from that, you have to study General Knowledge. Keep reading standard newspaper every day for General Awareness updates. You can follow online educators for normal guidance. Keep yourself physically fit.
Moreover, practice previous years questions paper.
It is a good that you have set up a target for NDA just after completing Xth exams. The minimum qualification for NDA is 10+2, so you can prepare yourself well simultaneously with your further studies.
Doing 10+2 with Science(Physics and Maths) would be a wiser option as it gives you the options of Army, Air Force and Navy. Phy and Maths are compulsory for Air Force and Naval Academy. Start brushing up your General Knowledge and do consult NCERT books on Indian History, Geography, Constitution etc of Xth level. At a later stage you may also join Coaching Center to prepare for SSB interview which is conducted after you qualify in the written exams.
Wish you all the best!