Hello.
There are two ways to take admission in BSc in geography , one is through merit based and other is through entrance exam based but second way is possible in less cases because only some Universities/colleges go through it now basic eligibility criteria for getting into BSc geography is that you must have completed your class 12 in any stream with minimum agreegate 50 percent from any recognised board ( this cutoff vary college to college).
There are some top colleges for BSc in geography in Kolkata , you can pick anyone and can apply for your course and rest of the process will be explained you by the college/University ,in general after application there will be merit list on which you admission will be done.

• Jadavpur University.
• Adamas University , Kolkata.
• Ashutosh college, Kolkata.
• Shri shikshayatan college, Kolkata.
• Vivekanand college, Kolkata.
• Rammohan college Kolkata, etc.

Best of luck.