Hi,
First of all, you will have to complete an LLB programme from BCI recognized institute. But a Law degree alone is not enough to become a practicing lawyer. After completing Law School, one must pass the All India Bar Exam (AIBE).Eligible persons are admitted as advocates on the rolls of the State Bar Councils. Those admitted as advocates by any State Bar Council are eligible for a Certificate of Enrolment. And practice as lawyer. As you want to become a criminal lawyer you can do LLM in Criminal Laws. You can also learn few trade secrets by joing a senior criminal lawyer. I hope it helps.