For a scientist post, you can choose many fields. You can do your engineering, you can do your MBBS or you can do B.Sc Science and then M.Sc Science. You can study Chemistry, Physics, Zoology. biochemistry and many more subjects. Once you complete your graduation in engineering or medicine or post graduation there are many universities or organisation who are engaged in research will offer the scientist post. To name a few such Organisations, Drdo, CSIR, Survey of India, ONGC, HAL.ISRO. All these Organisation will have scientist post. Basing on your interest and availability you can choose any field in which you want to study. Another advantage you will have here is while working you can improve your qualification also. So you can take a decision as per your requirement and see that you will be a scientist in coming days. All the best to you.