LLM after B.com? why LLB?

I'm in the final year of B.ocm right now. I'm giving an entrance exam for LLB. Please suggest the subjects and core areas I should study. Also I was wondering that if I'm going to be a graduate why can't I do Masters( i mean LLM)? why do I have to go all the way to do a 3 year Bachelors(LLB) again although I am going to be a graduate?