Hi. I really want to do my graduation in america. I am a medical student right now in 12th standard. I have heard that those who are opting for mbbs have to do bsc before then only they can do their mbbs.. is it correct? Is it necessary ?

In the US we don't use the term "MBBS," but rather "Medical Degree." It is a post-graduate degree, so yes, you would need to have a Bachelor's degree or equivalent from a 4-year college in order to apply to medical schools.