I think you are totally confused with the word 'Scientist' and of the impression that you have to do necessarily study Science subject to become a Scientist. It is totally a wrong conception. You have not provided your present course of study and age. Perhaps you are aware of the difference between Science and Humanities and I can presume that you are interested in Science subjects.
You have an option to study either Science or Humanities in your First year of Intermediate or XI standard. An option for Mathematics and Physical Sciences would help you to do your Graduation in Engineering through the respective State Technical Education authorities or JEE (in respect of IITs/NITs etc) or B Sc in a Degree college. If your are interested in Medicine or Life Sciences, you can opt for BiPC group.
Now coming to your choice to become a scientist, after completing your B E/B Tech degree, you can join as a Scientist in various Scientific and Research centres like CSIR, NRSA, DRDO, ISRO etc. If you want to do your B Sc, then you have pursue Master's degree in relevant field to become a Scientist as B E or M Sc are the eligible degrees for appointment of such posts. Same is the case with Medicine and Pharmaceutical research institutes. So, plan you study accordingly to realize your ambition.