Hello anchal

You can get a lota of answers ,tips and tricks on the website and each have their own personal opinions ,so follow that which suits you most . In my opinion first you should in serve the syllabus and should avoid doing anything other than syllabus .Since time left is very less so you should start doing previous year papers and nick tests . You have to give a lot of time in revision and attempting mock tests and previous years papers .Sine its lock down and most of the schools are closed so try giving it atleast 5-6 hours daily with full concentration . Personally i know one thing ,if you really want something ,you will get it

https://www.google.com/amp/s/school.careers360.com/articles/ntse-syllabus/amp

Best wishes