What can I do after 12th if I want to become a researcher/scientist?

The capacity to ask the appropriate questions and pursue issues that, if solved, maybe disruptive in nature is the most essential attribute of a scientist.
Most students become stuck as a result of a lack of clarity in defining an issue and then a lack of perseverance in seeing through the problem and taking on challenges that would lead them to their objectives.
Remember that while you will have mentors to assist you as a PhD student, no one will tell you what to do. You’re on your own and need to solve problems quickly.
A scientist’s normal path is to get a BSc (Physics, Chemistry, Biology, or Math), then a Master’s, and finally a Ph.D. You can also pursue an integrated Master’s degree and then continue on to a PhD program. Some students get a BTech and then pursue a PhD.

It doesn’t matter whatever road you choose; what counts is that you are clear about the issues you want to address and that you are determined to overcome them!