What will Ido for NSEc as I am in 11th standard and want to qualify this and all syllabus is from 12th

Take regular online practice tests: There are a number of online practice tests available for you to evaluate yourself.  Keep taking these tests, and ensure that you score a minimum of 95% in them.

Be tough with yourself: Remember again that you will be competing against the best. Be very tough on yourself, both in preparation and in marking.

Take advantage of online resources: There are a number of online resources available to supplement your study material; you just have to look for them. 

Be sure to study Organic chemistry: The Indian Chemistry Olympiad and the International Chemistry Olympiad ask a wide variety of topics from organic chemistry.  Some of the topics asked are basic reactions, nomenclature, stereochemistry, Sn1, Sn2, E1 and E2 reactions.

Visit forums: If you can’t attend special classes for your training, visit forums on chemistry and participate in the discussions.  They are normally very informative and will give you preparation pointers too.

Work hard: Finally, there is no substitute for consistent hard work.  It is only when you really apply yourself that you will be able to get the results you are aiming for.