The desired eligibility to pursue B.Sc Geology is that you should have completed your 10+2 in Physics, Chemistry, Biology/Mathematics with a minimum aggregate of 50%. Because you have studied Biology, you are eligible for B.Sc Geology. There are few Institutions which conduct entrance examinations and few Institutions conduct admissions based on merit.

Geology is a subject that deals with earth sciences concerned with solid rocks, water resources or natural satellites.
If you have decided to opt for geology further what you can go for is an undergraduate course in geology. You can pursue B.Sc. in geology from the recognised universities that provides this course. After that you can do major in any one of the topics of geology that fascinates you like Mineralogy, petrology, Hydrogeology etc. either in india or abroad. Geology is a subject that has something to research every time to touch a topic. There's always some open research project available in this field. If you don't want to go for higher studies then you can get job in agencies that work in this field like - GSI ( Geological Survey of India) , ONGC, SAIL, COAL India, GMDC ( Gujarat Mineral Development Corporation) , Govt. operated quarries etc.