Because you already have studied Physics and Chemistry in your 12th, you need to take up Biology as an additional subject from NIOS and appear for NEET. And aggregate of Physics and Chemistry from your 12th and Biology from your NIOS will be calculated and you should score an aggregate of 50% in these 3 subjects. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST.