You will need to either get a 3 years LLB degree or a 5 years integrated law degree to be eligible for becoming a lawyer. You have also the option to pursue a LLM in Corporate Law/Investment Law.
After you start practicing law, it does take a few years to feel comfortable and to gain experience dealing with certain types of clients. The best way to become a good corporate lawyer is to find another good corporate lawyer to work with.
If you have a good mentor and a steady stream of clients you should be able to be very competent in 3 to 5 years after class 12.