Hello dear..You  didn't mentioned from which College you want to pursue your course or from where you want to pursue your degree. In that case I can give only rough idea.

B.Sc Computer Science is a 3 year Undergraduate course, consist of 6 Semesters.

Minimum eligibility to appear for this course Is 10+2 or intermediate with minimum 50% marks from recognized board in science stream.

In most of the Universities admission is done on the basis of merit. University release the cutoff list.whearas some universities such as BHU demands for entrance exam. In most of the Universities Admission forms are available in the month of May and June. You can apply on the official page of concerning university.

Hope it helps

Good Luck