Just concentrate on your NCERT syllabus....If You want to join NDA. Please focus on mathematics of 11th and 12th.
Mathematics is very important for NDA. Because your first paper is mathematics and without getting passed in mathematics, second paper is not evaluated. So my suggestion is firstly prepare maths then focus on english and general knowledge etc...Little bit extra work in mathematics is needed...Read newspapers, gk books.. stay aware of happenigs in the world...Maintain good fitness....Try to participate in extra curricular activities and most importantly Improve ur english and try to evolve leadership qualities.