Yes, you can opt for LLB after b.com. the basic requirement is being a graduate student but you have to undergo examination for taking admission in  any law college.
To apply for this course you should complete bachelor degree in any discipline with minimum 50% marks. Fir sc/st candidates there is a relaxtion of 5% marks.
The admission for LLB course, is based on the performance for written examination  (Clat/Lsat/Nlsiu etc.) And group discussions and Interviews conducted by the institute.