Hi, Before rushing up to things you will have to plan your preparation systematically.
1st-Build your study schedule systematically which covers both NDA preparation as well as 12th boards.
2nd- You can brush your knowledge with class 10th social science books.
3rd- Brush your GK and for that you can read newspapers daily and gather information on sports, authors, books etc.
4th- Improve your English. A bit of daily practice can help in your 12th boards preparation as well as NDA preparation.
There is negative marking so you will have to avoid wrong answers or guesses.
Everything has to be systematically planned. good Luck.