Hi Kanni,

To become a scientist after getting into IITs, you will have to do research oriented works during your B.Tech course and have to write some good research articles in famous journals.

You should have passion about researching and finding new and innovative things. that's all. IITs have a much better platform than any other college for research oriented studies.

Refer here for more info - https://engineering.careers360.com/colleges/list-of-top-iit-colleges-in-india