Science

Hello,
If you want to pursue M.Sc from IITs/IISc through IIT JAM then you must have 55% aggregate marks at B.Sc. Level. If you satisfied the complete criteria then you can apply for JAM. If you have done your B.Sc in chemistry then I will suggest you prepare for IIT JAM Chemistry: https://goo.gl/F8vLLR
The basic criteria for JAM given below: -
IIT JAM 2019 is open to all nationals (Indian/Foreign). There is no age restriction, i.e. there is no IIT JAM age eligibility, regardless of your current age, you can apply for JAM. To apply for admission to the desired programme, a candidate is required to qualify in the corresponding Test Paper and also satisfy the Minimum Educational Qualifications (MEQs) and Eligibility Requirements (ERs) of the respective academic programme. The candidates who have either appeared or are due to appear in the final examination of their qualifying degree in 2019 are also eligible to appear in the test.

Economics is however regarded as a science subject but the field is different. Though economics is based on using science calculations but they are completely different field. If your future is concerned with arts then you will have to choose economics, if your main focus is on science then you may take science and can fall into the field of micro or macroeconomics as per your interest.
If you choose to study economics then you might deal with economics itself majorly but if you choose then I think you must focus on 12th and 11th grade but according to my knowledge you may try to occupy a degree in actuarial science which requires proficiency of maths, science, statistics, accounting or business.