How many mininum marks should i secure to get a seat at NISER in the NEST examination in open catergory .? How should i prepare for it?

Hi, This is extremely difficult to predict as it depends upon the number of applicants and the difficulty levels of the question paper. The ranking methodology is pretty complicated.  It is somewhat like this

Total score of each candidate will be the direct sum of the score obtained in the general section and his/her best three scores in the subject sections. Every candidates has to score above Section–wise Minimum Admissible Score (SMAS) which is 20% of the average of the best 100 scores in that section. In addition each candidate must score equal to or more than respective SMAS in the General section as well as in at least three subject sections which will be counted for merit list calculation. The best three subject scores will be taken into account for a candidate who secures SMAS in all four subject sections. Plus to get a merit rank, every candidate must score equal to or above a total Minimum Admissible Score (MAS) in addition to fulfilling SMAS conditions. For NEST, 2017 it was 90 marks. So if you score less than 90, you will not be allotted any merit rank. For SC/ST/Pwd the MAS is 50% while for general /OBC it is 90%.

So to have a chance at a seat, past data suggests you need more than 100 marks. As to how to prepare for the exam:

Check here https://www.nestexam.in. Check the syllabus first. Then check the past question papers given here. The weightage of topics will be known if you analyse the papers properly. You could then segregate the topics into easy, difficult and to learn. Revise the easy one with one difficult and one to be learnt topics so that you complete. solve those question papers and you should perform well. All the best!