Hi, You ought to have BSc or BTech degree before getting into research field.
Research field signifies you are going to do doctor of philosophy (PhD) in that particular area. So, after 12th, you should go for B Sc degree in Physics and then go for National Eligibility Test (NET), Joint Entrance Screening Test (JEST) and GATE Physics exam for getting admissions into research centers (GATE Physics gets you into I I T's) like T I F R, H R I. These are great research institutes for Theoretical Physics. Above mentioned exams can only be applied for Integrated PhD (except GATE). Otherwise, you ought to have M Sc degree too to go for just PhD's.

This is the process you will have to follow.
Good Luck.