I have PCM in class 12th. What should I do to switch to the field of law? What exams should I give and how to prepare for them?

You have to give a national level entrance exam for law college which is called CLAT (common law admission test). For that, you need to study the sections, have to do some case studies. Try to take a look of how the arguments are going on in the court. All the best for your future.