You can do LLB either after class XIIth or after graduation in any subject. If you do it after class XIIth, you will study for 5 years and after graduation, the duration is 3 years. But most of the institutes in India except those in Maharashtra now offer only the 5 years course after class XIIth.

Yes there is a requirement of obtaining a certain marks in your qualifying examination. It varies from Institute to Institute. For example, for admission to National Law School of India University, Bangalore, you will need at least 50% marks in aggregate in your class XII examination. Some institutes ask for 45% marks in aggregate.