If you want to become a scientist with engineering background then after 12th do engineering. Maintain really good academics throughout your engineering career.You can get selected in R&D of good private companies if you do engineering from a good college through placements. If you want to become a scientist in government organisation you can try for BARC,ISRO,DRDO,NPCIL,etc. All these government organisations demand very good academic records. Some of them hire through GATE exam after engineering and some conduct their own exam. It will be good if you maintain 70% throughout your engineering