Well, I would say something different from the others. You don't have to make biology your major in undergrad. In fact medical schools in the United States are looking for applicants that have diverse backgrounds and have different skills. There are medical students with literature, psychology, engineering and even music backgrounds. So don't limit yourself.
choose biology in your 12th stad and then write entrance exams to get MBBS seat. choose cardiology as specialization.
To be a cardiologist opt for biology in your pre-university and acquire good scores
once that is done you can opt for MBBS with cardiology as major in an institute of your choice
after MBBS you can choose to specialize in cardiology by doing an MS or MD
Best of luck :)